xfs: fully initialise temp leaf in xfs_attr3_leaf_compact

xfs_attr3_leaf_compact() uses a temporary buffer for compacting the
the entries in a leaf. It copies the the original buffer into the
temporary buffer, then zeros the original buffer completely. It then
copies the entries back into the original buffer.  However, the
original buffer has not been correctly initialised, and so the
movement of the entries goes horribly wrong.

Make sure the zeroed destination buffer is fully initialised, and
once we've set up the destination incore header appropriately, write
is back to the buffer before starting to move entries around.

While debugging this, the _d/_s prefixes weren't sufficient to
remind me what buffer was what, so rename then all _src/_dst.

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