XFS: xfs_trans_add_item() - don't assign in ASSERT() when compare is intended

It looks to me like the two ASSERT()s in xfs_trans_add_item() really
want to do a compare (==) rather than assignment (=).
This patch changes it from the latter to the former.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Ben Myers <bpm@sgi.com>