ceph: add acl, noacl options for cephfs mount

Make the 'acl' option dependent on having ACL support compiled in.  Make
the 'noacl' option work even without it so that one can always ask it to
be off and not error out on mount when it is not supported.

Signed-off-by: Guangliang Zhao <lucienchao@gmail.com>
Signed-off-by: Sage Weil <sage@inktank.com>