MFC r318819: MFV r316908: 7541 zpool import/tryimport ioctl returns ENOMEM because...

MFC r318819: MFV r316908: 7541 zpool import/tryimport ioctl returns ENOMEM because provided buffer is too small for config

illumos/illumos-gate@8b65a70b763232c90a91f31eb2010314c02ed338
https://github.com/illumos/illumos-gate/commit/8b65a70b763232c90a91f31eb2010314c02ed338

https://www.illumos.org/issues/7541
  When calling zpool import, zpool does a few ioctls to ZFS.
  zpool allocates a buffer in userland and passes it to the kernel so that ZFS
  can copy info into it. ZFS will use it to put the nvlist that describes the
  pool configuration.
  If the allocated buffer is too small, ZFS will return ENOMEM and the call will
  have to be redone. This wastes CPU time and slows down the import process. This
  happens very often for the ZFS_IOC_POOL_TRYIMPORT call.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Pavel Zakharov <pavel.zakharov@delphix.com>