MFV r303081: 7163 ztest failures due to excess error injection

MFV r303081: 7163 ztest failures due to excess error injection

illumos/illumos-gate@f34284d835bc555f987c1310df46c034c3101155
https://github.com/illumos/illumos-gate/commit/f34284d835bc555f987c1310df46c034c
3101155

https://www.illumos.org/issues/7163
  Running zloop from zfs-precommit hit this assertion:
       *panicstr/s
  0xfffffd7fd7419370: assertion failed for thread 0xfffffd7fe29ed240,
  thread-id 577: parent != NULL, file ../../../uts/common/fs/zfs/dbuf.c, line
  1827
       $c
  libc.so.1`_lwp_kill+0xa()
  libc.so.1`_assfail+0x182(fffffd7ffb1c29fa, fffffd7ffb1cc028, 723)
  libc.so.1`assfail+0x19(fffffd7ffb1c29fa, fffffd7ffb1cc028, 723)
  libzpool.so.1`dbuf_dirty+0xc69(10e3bc10, 3601700)
  libzpool.so.1`dbuf_dirty+0x61e(10c73640, 3601700)
  libzpool.so.1`dbuf_dirty+0x61e(10e28280, 3601700)
  libzpool.so.1`dmu_buf_will_fill+0x64(10e28280, 3601700)
  libzpool.so.1`dmu_write+0x1b6(2c7e640, d, 400000002e000000, 200, 3717b40,
  3601700)
  ztest_replay_write+0x568(4950d0, 3717a80, 0)
  ztest_write+0x125(4950d0, d, 400000002e000000, 200, 413f000)
  ztest_io+0x1bb(4950d0, d, 400000002e000000)
  ztest_dmu_write_parallel+0xaa(4950d0, 6)
  ztest_execute+0x83(1, 420c98, 6)
  ztest_thread+0xf4(6)
  libc.so.1`_thrp_setup+0x8a(fffffd7fe29ed240)
  libc.so.1`_lwp_start()
  This is another manifestation of ECKSUM in ztest:
  The lowest level ancestor that’s in memory is the L8 (topmost). The L7
  ancestor is blkid 0x10:
       ::dbufs -O 0x2c7e640 -o d -l 7 |::dbuf
  addr object lvl blkid holds os
  600be50 d 7 4 1 ztest/ds_6
  719d880 d 7 0 4 ztest/ds_6

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>