8021 ARC buf data scatter-ization

8021 ARC buf data scatter-ization
8100 8021 seems to cause random BAD TRAP: type=d (#gp General protection)

illumos/illumos-gate@770499e185d15678ccb0be57ebc626ad18d93383
https://github.com/illumos/illumos-gate/commit/770499e185d15678ccb0be57ebc626ad18d93383

https://www.illumos.org/issues/8021
  The ARC buf data project (known simply as "ABD" since its genesis in the ZoL
  community) changes the way the ARC allocates `b_pdata` memory from using linear
  `void *` buffers to using scatter/gather lists of fixed-size 1KB chunks. This
  improves ZFS's performance by helping to defragment the address space occupied
  by the ARC, in particular for cases where compressed ARC is enabled. It could
  also ease future work to allocate pages directly from `segkpm` for minimal-
  overhead memory allocations, bypassing the `kmem` subsystem.
  This is essentially the same change as the one which recently landed in ZFS on
  Linux, although they made some platform-specific changes while adapting this
  work to their codebase:
  1. Implemented the equivalent of the `segkpm` suggestion for future work
  mentioned above to bypass issues that they've had with the Linux kernel memory
  allocator.
  2. Changed the internal representation of the ABD's scatter/gather list so it
  could be used to pass I/O directly into Linux block device drivers. (This
  feature is not available in the illumos block device interface yet.)

https://www.illumos.org/issues/8100
  My supermicro system is getting random BAD TRAP: type=d (#gp General
  protection) at about the stage where ZFS filesystems are mounted - usually
  console login prompt is already present but the services are still starting.
  After backing out 8021, the boot is completed and no panics do occur.
  Machine does dump, however savecore fails:
  savecore: bad magic number baddcafe
  I can get more data out with boot -k, if needed.
  # psrinfo -vp
  The physical processor has 4 cores and 8 virtual processors (0-7)
    The core has 2 virtual processors (0 4)
    The core has 2 virtual processors (1 5)
    The core has 2 virtual processors (2 6)
    The core has 2 virtual processors (3 7)
      x86 (GenuineIntel 306C3 family 6 model 60 step 3 clock 3500 MHz)
        Intel(r) Xeon(r) CPU E3-1246 v3 @ 3.50GHz

  # prtconf -m
  32657

  $ zpool status
    pool: rpool
   state: ONLINE
    scan: none requested
  config:

          NAME        STATE     READ WRITE CKSUM
          rpool       ONLINE       0     0     0
            raidz1-0  ONLINE       0     0     0
              c3t0d0  ONLINE       0     0     0
              c3t1d0  ONLINE       0     0     0

Reviewed by: Matthew Ahrens mahrens@delphix.com
Reviewed by: George Wilson george.wilson@delphix.com
Reviewed by: Paul Dagnelie pcd@delphix.com
Reviewed by: John Kennedy john.kennedy@delphix.com
Reviewed by: Prakash Surya prakash.surya@delphix.com
Reviewed by: Prashanth Sreenivasa pks@delphix.com
Reviewed by: Pavel Zakharov pavel.zakharov@delphix.com
Reviewed by: Chris Williamson chris.williamson@delphix.com
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Dan Kimmel <dan.kimmel@delphix.com>