ceph: fix cap release race

ceph_encode_inode_release() can race with ceph_open() and release
caps wanted by open files. So it should call __ceph_caps_wanted()
to get the wanted caps.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>