quota: port quota helpers mount ids

Port the is_quota_modification() and dqout_transfer() helper to type
safe vfs{g,u}id_t. Since these helpers are only called by a few
filesystems don't introduce a new helper but simply extend the existing
helpers to pass down the mount's idmapping.

Note, that this is a non-functional change, i.e. nothing will have
happened here or at the end of this series to how quota are done! This
a change necessary because we will at the end of this series make
ownership changes easier to reason about by keeping the original value
in struct iattr for both non-idmapped and idmapped mounts.

For now we always pass the initial idmapping which makes the idmapping
functions these helpers call nops.

This is done because we currently always pass the actual value to be
written to i_{g,u}id via struct iattr. While this allowed us to treat
the {g,u}id values in struct iattr as values that can be directly
written to inode->i_{g,u}id it also increases the potential for
confusion for filesystems.

Now that we are have dedicated types to prevent this confusion we will
ultimately only map the value from the idmapped mount into a filesystem
value that can be written to inode->i_{g,u}id when the filesystem
actually updates the inode. So pass down the initial idmapping until we
finished that conversion at which point we pass down the mount's
idmapping.

Since struct iattr uses an anonymous union with overlapping types as
supported by the C standard, filesystems that haven't converted to
ia_vfs{g,u}id won't see any difference and things will continue to work
as before. In other words, no functional changes intended with this
change.

Link: https://lore.kernel.org/r/20220621141454.2914719-7-brauner@kernel.org


Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>
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: Jan Kara <jack@suse.cz>
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>