os: add a field indicate xattr only one chunk for set xattr.

os: add a field indicate xattr only one chunk for set xattr.

Because the limit of osd backend, we split xattr into a lot of small
pieces. When set xatt, it should remove the old pieces.
But for some xattr especially for inline xattr, we know it only
one chunk. So it can skip this remove operation.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>