ceph: drop CAP_LINK_SHARED when sending "link" request to MDS

To handle "link" request, the MDS need to xlock inode's linklock,
which requires revoking any CAP_LINK_SHARED.

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