fs: introduce tiny iattr ownership update helpers

Nearly all fileystems currently open-code the same checks for
determining whether the i_{g,u}id fields of an inode need to be updated
and then updating the fields.

Introduce tiny helpers i_{g,u}id_needs_update() and i_{g,u}id_update()
that wrap this logic. This allows filesystems to not care about updating
inode->i_{g,u}id with the correct values themselves instead leaving this
to the helpers.

We also get rid of a lot of code duplication and make it easier to
change struct iattr in the future since changes can be localized to
these helpers.

And finally we make it hard to conflate k{g,u}id_t types with
vfs{g,u}id_t types for filesystems that support idmapped mounts.

In the following patch we will port all filesystems that raise
FS_ALLOW_IDMAP to use the new helpers. However, the ultimate goal is to
convert all filesystems to make use of these helpers.

All new helpers are nops on non-idmapped mounts.

Link: https://lore.kernel.org/r/20220621141454.2914719-5-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

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