xfs: use generic posix ACL infrastructure

Also don't bother to set up a .get_acl method for symlinks as we do not
support access control (ACLs or even mode bits) for symlinks in Linux,
and create inodes with the proper mode instead of fixing it up later.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>