xfs: remove buftarg hash for external devices

For RT and external log devices, we never use hashed buffers on them
now.  Remove the buftarg hash tables that are set up for them.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>

fs/xfs/linux-2.6/xfs_buf.c

@@ -1458,7 +1458,11 @@ xfs_alloc_bufhash(
 {
 	unsigned int		i;
 
-	btp->bt_hashshift = external ? 3 : 12;	/* 8 or 4096 buckets */
+	if (external) {
+		btp->bt_hash = NULL;
+		return;
+	}
+	btp->bt_hashshift = 12;	/* 4096 buckets */
 	btp->bt_hash = kmem_zalloc_large((1 << btp->bt_hashshift) *
 					 sizeof(xfs_bufhash_t));
 	for (i = 0; i < (1 << btp->bt_hashshift); i++) {