xfs: split out attribute listing code into separate file

The attribute listing code is not used by userspace, so like the
directory readdir code, split it out into a separate file to
minimise the differences between the filesystem shared with libxfs
in userspace.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>