ocfs2/xattr: Remove extend_trans call and add its credits from the beginning

Actually, when setting a new xattr value, we know it from the very
beginning, and it isn't like the extension of bucket in which case
we can't figure it out. So remove ocfs2_extend_trans in that function
and calculate it before the transaction. It also relieve acl operation
from the worry about the side effect of ocfs2_extend_trans.

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