ceph: simplify ceph_{get,init}_acl

 - ->get_acl only gets called after we checked for a cached ACL, so no
   need to call get_cached_acl again.
 - no need to check IS_POSIXACL in ->get_acl, without that it should
   never get set as all the callers that set it already have the check.
 - you should be able to use the full posix_acl_create in CEPH

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Sage Weil <sage@inktank.com>