fs: Refuse uid/gid changes which don't map into s_user_ns

Add checks to notify_change to verify that uid and gid changes
will map into the superblock's user namespace. If they do not
fail with -EOVERFLOW.

This is mandatory so that fileystems don't have to even think
of dealing with ia_uid and ia_gid that

--EWB Moved the test from inode_change_ok to notify_change

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>