xfs: fully initialise temp leaf in xfs_attr3_leaf_unbalance

xfs_attr3_leaf_unbalance() uses a temporary buffer for recombining
the entries in two leaves when the destination leaf requires
compaction. The temporary buffer ends up being copied back over the
original destination buffer, so the header in the temporary buffer
needs to contain all the information that is in the destination
buffer.

To make sure the temporary buffer is fully initialised, once we've
set up the temporary incore header appropriately, write is back to
the temporary buffer before starting to move entries around.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>