fs: use mount types in iattr

Add ia_vfs{g,u}id members of type vfs{g,u}id_t to struct iattr. We use
an anonymous union (similar to what we do in struct file) around
ia_{g,u}id and ia_vfs{g,u}id.

At the end of this series ia_{g,u}id and ia_vfs{g,u}id will always
contain the same value independent of whether struct iattr is
initialized from an idmapped mount. This is a change from how this is
done today.

Wrapping this in a anonymous unions has a few advantages. It allows us
to avoid needlessly increasing struct iattr. Since the types for
ia_{g,u}id and ia_vfs{g,u}id are structures with overlapping/identical
members they are covered by 6.5.2.3/6 of the C standard and it is safe
to initialize and access them.

Filesystems that raise FS_ALLOW_IDMAP and thus support idmapped mounts
will have to use ia_vfs{g,u}id and the associated helpers. And will be
ported at the end of this series. They will immediately benefit from the
type safe new helpers.

Filesystems that do not support FS_ALLOW_IDMAP can continue to use
ia_{g,u}id for now. The aim is to convert every filesystem to always use
ia_vfs{g,u}id and thus ultimately remove the ia_{g,u}id members.

Link: https://lore.kernel.org/r/20220621141454.2914719-4-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>

include/linux/mnt_idmapping.h

@@ -21,6 +21,11 @@ typedef struct {
 	gid_t val;
 } vfsgid_t;
 
+static_assert(sizeof(vfsuid_t) == sizeof(kuid_t));
+static_assert(sizeof(vfsgid_t) == sizeof(kgid_t));
+static_assert(offsetof(vfsuid_t, val) == offsetof(kuid_t, val));
+static_assert(offsetof(vfsgid_t, val) == offsetof(kgid_t, val));
+
 #ifdef CONFIG_MULTIUSER
 static inline uid_t __vfsuid_val(vfsuid_t uid)
 {