ceph: update the mtime when truncating up

If we have Fx caps, and the we're truncating the size to be larger, then
we'll cache the size attribute change, but the mtime won't be updated.

Move the size handling before the mtime, and add ATTR_MTIME to ia_valid
in that case to make sure the mtime also gets updated.

This fixes xfstest generic/313.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>