MFV r275542:

If a dnode has a spill block and there is an error while accessing
a data block then traverse_dnode() loses information about that error
and returns a status of visiting the spill block.

This issue is discovered by Spectra Logic.

Illumos issue:
    5311 traverse_dnode may report success when it should not

Original author:	gibbs
MFC after:		2 weeks
MFV r275543:

Remove always true tests for ds->ds_phys' presence.

Clean up assertions in dsl_dataset_disown.

Remove unreachable code in dsl_dataset_disown().

Illumos issue:
    5310 Remove always true tests for non-NULL ds->ds_phys

MFC after:	2 weeks
MFV r275544:

Clean up some duplicated code in dnode_sync() around freeing spill blocks.

Illumos issue:
    5350 clean up code in dnode_sync()

MFC after:	2 weeks
MFV r275545:

If zio_checksum_error() returns other than ECKSUM (e.g. EINVAL), it does not
fill in the "zio_bad_cksum_t *info" parameter. Caller should not attempt to
use it in this case.

Illumos issue:
    5348 zio_checksum_error() only fills in info if ECKSUM

MFC after:	2 weeks
MFV r275546:

Reduce scrub activities when system there is enough dirty data, namely when
dirty data is more than zfs_vdev_async_write_active_min_dirty_percent (once
we start to increase the number of concurrent async writes).

While there also correct rounding error which would make scrub end up
pausing for (zfs_txg_timeout + 1) seconds instead of the desired
zfs_txg_timeout seconds.

Illumos issue:
    5351 scrub goes for an extra second each txg
    5352 scrub should pause when there is some dirty data

MFC after:	2 weeks
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    4181 zfs(1m): 'zfs allow' examples in the man page are outdated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