ceph: use CEPH_MDS_OP_RMXATTR request to remove xattr

Setxattr with NULL value and XATTR_REPLACE flag should be equivalent
to removexattr. But current MDS does not support deleting vxattrs through
MDS_OP_SETXATTR request. The workaround is sending MDS_OP_RMXATTR request
if setxattr actually removs xattr.

Signed-off-by: Yan, Zheng <zyan@redhat.com>