MFV r316901:

MFV r316901:
7730 libzfs`add_config() leaks config nvl when reading spare/l2cache devices

illumos/illumos-gate@105686550ee9cbf5d033166a8a2a5a763667d436
https://github.com/illumos/illumos-gate/commit/105686550ee9cbf5d033166a8a2a5a763667d436

https://www.illumos.org/issues/7730
  antares:root:~# mdb /usr/sbin/zpool
  > ::sysbp _exit
  > ::run import
     pool: data
       id: 2093977168778024605
    state: ONLINE
   action: The pool can be imported using its name or numeric identifier.
   config:

          data        ONLINE
            c6t0d0    ONLINE
            c6t1d0    ONLINE
          cache
            c6t2d0
  mdb: stop on entry to _exit
  mdb: target stopped at:
  0xfee556ba:     nop
  mdb: You've got symbols!
  Loading modules: [ ld.so.1 libumem.so.1 libc.so.1 libtopo.so.1 libavl.so.1
  libnvpair.so.1 ]
  > ::findleaks -d
  BYTES             LEAKED VMEM_SEG CALLER
  4096                  10 fda7b000 MMAP
  8192                   1 fea8d000 MMAP
  8192                   1 fe76d000 MMAP
  8192                   1 fe66e000 MMAP
  4096                   1 fe570000 MMAP
  8192                   1 fe470000 MMAP
  4096                   1 fe372000 MMAP
  4096                   1 fe273000 MMAP

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Yuri Pankov <yuri.pankov@nexenta.com>