btrfs: optimize simple reads in btrfsic_map_block

Pass a smap into __btrfs_map_block so that the usual case of a read that
doesn't require parity raid recovery doesn't need an extra memory
allocation for the btrfs_io_context.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/check-integrity.c

@@ -1459,13 +1459,13 @@ static int btrfsic_map_block(struct btrfsic_state *state, u64 bytenr, u32 len,
 	struct btrfs_fs_info *fs_info = state->fs_info;
 	int ret;
 	u64 length;
-	struct btrfs_io_context *multi = NULL;
+	struct btrfs_io_context *bioc = NULL;
+	struct btrfs_io_stripe smap, *map;
 	struct btrfs_device *device;
 
 	length = len;
-	ret = btrfs_map_block(fs_info, BTRFS_MAP_READ,
-			      bytenr, &length, &multi, mirror_num);
-
+	ret = __btrfs_map_block(fs_info, BTRFS_MAP_READ, bytenr, &length, &bioc,
+				NULL, &mirror_num, 0);
 	if (ret) {
 		block_ctx_out->start = 0;
 		block_ctx_out->dev_bytenr = 0;
@@ -1478,21 +1478,26 @@ static int btrfsic_map_block(struct btrfsic_state *state, u64 bytenr, u32 len,
 		return ret;
 	}
 
-	device = multi->stripes[0].dev;
+	if (bioc)
+		map = &bioc->stripes[0];
+	else
+		map = &smap;
+
+	device = map->dev;
 	if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state) ||
 	    !device->bdev || !device->name)
 		block_ctx_out->dev = NULL;
 	else
 		block_ctx_out->dev = btrfsic_dev_state_lookup(
 							device->bdev->bd_dev);
-	block_ctx_out->dev_bytenr = multi->stripes[0].physical;
+	block_ctx_out->dev_bytenr = map->physical;
 	block_ctx_out->start = bytenr;
 	block_ctx_out->len = len;
 	block_ctx_out->datav = NULL;
 	block_ctx_out->pagev = NULL;
 	block_ctx_out->mem_to_free = NULL;
 
-	kfree(multi);
+	kfree(bioc);
 	if (NULL == block_ctx_out->dev) {
 		ret = -ENXIO;
 		pr_info("btrfsic: error, cannot lookup dev (#1)!\n");