ceph: show non-default options only

Don't pollute /proc/mounts with default options (presently these are
dcache, nofsc and acl).  Leave the acl/noacl however - it's a bit of
a special case due to CONFIG_CEPH_FS_POSIX_ACL.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>