xfs: fix node forward in xfs_node_toosmall

xfs: fix node forward in xfs_node_toosmall

When a node is considered for a merge with a sibling, it overwrites the
sibling pointers of the original incore nodehdr with the sibling's
pointers.  This leads to loop considering the original node as a merge
candidate with itself in the second pass, and so it incorrectly
determines a merge should occur.)

Ported from equivalent kernel commit 997def25.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>