ceph: new mount option that specifies fscache uniquifier

Current ceph uses FSID as primary index key of fscache data. This
allows ceph to retain cached data across remount. But this causes
problem (kernel opps, fscache does not support sharing data) when
a filesystem get mounted several times (with fscache enabled, with
different mount options).

The fix is adding a new mount option, which specifies uniquifier
for fscache.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>