ceph: allow arbitrary security.* xattrs

Most filesystems don't limit what security.* xattrs can be set or
fetched. I see no reason that we need to limit that on cephfs either.

Drop the special xattr handler for "security." xattrs, and allow the
"other" xattr handler to handle security xattrs as well.

In addition to fixing xfstest generic/093, this allows us to support
per-file capabilities (a'la setcap(8)).

Link: https://tracker.ceph.com/issues/41135
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>