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

setxattr() should remove xattr if parameter 'value' is NULL. To
distinguish it from the zero-length value case, introduce a new
flag CEPH_XATTR_REMOVE for the setxattr request.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1216 from ceph/wip-null-xattr

mds: remove xattr when null value is given to setxattr()

Reviewed-by: Sage Weil <sage@inktank.com>