MFV r275547:

Port Illumos 'zfs allow' examples update.  While I'm there also fix
a typo.

Illumos issue:
    4181 zfs(1m): 'zfs allow' examples in the man page are outdated

MFC after:	2 weeks
MFV r275548:

Verify that the block pointer is structurally valid, before attempting to
read it in.  It can only be invalid in the case of a ZFS bug, but this
change will help identify such bugs in a more transparent way, by
panic'ing with a relevant message, rather than indexing off the end of an
array or something.

Illumos issue:
    5349 verify that block pointer is plausible before reading

MFC after:	2 weeks