Code cleanup after sharing zfs_write between platforms

[ghost]

Motivation and Context

Housekeeping

Description

• Factor uid, gid, and projid out of loop in zfs_write
• Remove redundant oid parameter to update_pages
  The oid comes from the znode we are already passing.
• Prune unused alias for dmu_assign_arcbuf_by_dbuf
• Move uio_prefaultpages definition to uio.h
• Const some unchanging variables in zfs_write
  Show that these values will not be changing later.

How Has This Been Tested?

ZTS

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

This function is so big, that I doubt there will be many free registers to store those values across the loop iterations.

Generally I think this per-block loop in ZFS is over-aggressive, causing more than necessary separate log records, which is a downside now that zil code is able to properly split the large writes between multiple blocks. So I would prefer to see here less cycle iterations rather than optimizations to make it behave.

[behlendorf]

Merged, with the exception of the "Prune unused alias for dmu_assign_arcbuf_by_dbuf" commit. We actually have one external consumer, Lustre, which uses this alias. For now, I'd like to keep it to preserve compatibility.