ceph: remove xattr when null value is given to setxattr()

For the setxattr request, introduce a new flag CEPH_XATTR_REMOVE
to distinguish null value case from the zero-length value case.

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

diff --cc fs/ceph/xattr.c
Simple merge

diff --cc include/linux/ceph/ceph_fs.h
Simple merge