ceph: fix ceph_flock()

VFS does not directly pass the flock's operation to the filesystem's
flock callback directly. It translates the flock's operation to posix
lock's command.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
ceph: forbid mandatory file lock

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>