btrfs: use bvec_kmap_local in btrfs_csum_one_bio

Using local kmaps slightly reduces the chances to stray writes, and
the bvec interface cleans up the code a little bit.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/file-item.c

@@ -708,12 +708,12 @@ blk_status_t btrfs_csum_one_bio(struct btrfs_inode *inode, struct bio *bio,
 				index = 0;
 			}
 
-			data = kmap_atomic(bvec.bv_page);
-			crypto_shash_digest(shash, data + bvec.bv_offset
-					    + (i * fs_info->sectorsize),
+			data = bvec_kmap_local(&bvec);
+			crypto_shash_digest(shash,
+					    data + (i * fs_info->sectorsize),
 					    fs_info->sectorsize,
 					    sums->sums + index);
-			kunmap_atomic(data);
+			kunmap_local(data);
 			index += fs_info->csum_size;
 			offset += fs_info->sectorsize;
 			this_sum_bytes += fs_info->sectorsize;