xlog_find_tail() currently leaks a bp on one error path.

There is no error target, so manually free the bp before
returning the error.

Found by Coverity.
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NMark Tinguely <tinguely@sgi.com>
Signed-off-by: NBen Myers <bpm@sgi.com>