MFC r305327: MFV r303080: 6451 ztest fails due to checksum errors

MFC r305327: MFV r303080: 6451 ztest fails due to checksum errors

illumos/illumos-gate@f9eb9fdf196b6ed476e4ffc69cecd8b0da3cb7e7
https://github.com/illumos/illumos-gate/commit/f9eb9fdf196b6ed476e4ffc69cecd8b0d
a3cb7e7

https://www.illumos.org/issues/6451
  Sometimes ztest fails because zdb detects checksum errors. e.g.:
  Traversing all blocks to verify checksums and verify nothing leaked ...
  zdb_blkptr_cb: Got error 50 reading <71, 47, 0, 8000160> DVA0=<0:1cc2000:
  180000> [L0 other uint64[]] sha256 uncompressed LE contiguou
  s unique single size=100000L/100000P birth=271L/271P fill=1
  cksum=c5a3e27d1ed0f894:843bca3a5473c4bf:f76a19b6830a2e4:91292591613a12bf --
  skipping
  zdb_blkptr_cb: Got error 50 reading <71, 47, 0, 800000180> DVA0=<0:ce16800:
  180000> [L0 other uint64[]] sha256 uncompressed LE contigu
  ous unique single size=100000L/100000P birth=840L/840P fill=1
  cksum=5d018f3d061e17f3:6d1584784587bf63:2805a74a0ce37369:ba68a214806c7e75
  -- skipping
  zdb_blkptr_cb: Got error 50 reading <71, 47, 0, 1000000360> DVA0=<0:10d37400:
  180000> [L0 other uint64[]] sha256 uncompressed LE conti
  guous unique single size=100000L/100000P birth=904L/904P fill=1
  cksum=fa1e11d4138bd14b:86c9488c444473e3:f31e43c72e72e46b:e3446472d1174d
  ba -- skipping
  zdb_blkptr_cb: Got error 50 reading <71, 47, 0, 400000002c0> DVA0=<0:127ef400:
  180000> [L0 other uint64[]] sha256 uncompressed LE cont
  iguous dedup single size=100000L/100000P birth=549L/549P fill=1
  cksum=30e14955ebf13522:66dc2ff8067e6810:4607e750abb9d3b3:6582b8af909fcb
  58 -- skipping
  zdb_blkptr_cb: Got error 50 reading <657, 5, 0, 1c0> DVA0=<0:1a180400:180000>
  [L0 other uint64[]] fletcher4 uncompressed LE contiguou
  s unique single size=100000L/100000P birth=1091L/1091P fill=1 cksum=a6cf1e50:
  29b3bd01c57e5:36779b914035db9a:db61cdcf6bec56f0 -- skippin
  g
  The problem is that ztest_fault_inject() can inject multiple faults into the
  same block. It is designed such that it can inject errors on all leafs of a
  RAID-Z or mirror, but for a given range of offsets, it will only inject errors

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Jorgen Lundman <lundman@lundman.net>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Matthew Ahrens <mahrens@delphix.com>