ceph: fix null pointer dereference in discard_cap_releases()

send_mds_reconnect() may call discard_cap_releases() after all
release messages have been dropped by cleanup_cap_releases()

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>
ceph: check buffer size in ceph_vxattrcb_layout()

If buffer size is zero, return the size of layout vxattr. If buffer
size is not zero, check if it is large enough for layout vxattr.

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