xfs: use setattr_copy to set VFS file attributes

A few weeks ago, Filipe Manana reported[1] that the new generic/673 test
fails on btrfs because btrfs is more aggressive about dropping the
setgid bit when reflinking into a file.  After some more digging, we
discovered that btrfs calls the VFS helpers to handle updating VFS
inode attributes, whereas XFS has open-coded logic dating from ~1997
that have not been kept up to date.

A few days later, Andrey Zhadchenko reported[2] that XFS can mistakenly
clear S_ISUID and S_ISGID on idmapped mounts.  After further discussion,
it was pointed out that the VFS already handles all these fiddly file
mode changes, and that it was the XFS implementation that is out of
date.

Both of these reports resolve to the same cause, which is that XFS needs
to call setattr_copy to update i_mode instead of doing it directly.
This series replaces all of our bespoke code with VFS calls to fix the
problem and reduce the size of the codebase by ~70 lines.

[1] https://lore.kernel.org/linux-xfs/CAL3q7H47iNQ=Wmk83WcGB-KBJVOEtR9+qGczzCeXJ9Y2KCV25Q@mail.gmail.com/
[2] https://lore.kernel.org/linux-xfs/20220221182218.748084-1-andrey.zhadchenko@virtuozzo.com/

v2: fix Fixes tag, add review tags
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