ocfs2/xattr: Only extend xattr bucket in need.

When the first block of a bucket is filled up with xattr
entries, we normally extend the bucket. But if we are
just replace one xattr with small length, we don't need
to extend it. This is important since we will calculate
what we need before the transaction and in this situation
no resources will be allocated.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>