btrfs: remove shadowing variables in __btrfs_map_block

1) We can safely use the function's 'i'. Fixes warning

fs/btrfs/volumes.c:5257:7: warning: declaration of 'i' shadows a previous local
fs/btrfs/volumes.c:4951:6: warning: shadowed declaration is here

2) A local variable duplicates name of an argument, we can use the value
directly. Fixes warning

fs/btrfs/volumes.c:5433:8: warning: declaration of 'length' shadows a parameter
fs/btrfs/volumes.c:4935:27: warning: shadowed declaration is here
Signed-off-by: NDavid Sterba <dsterba@suse.cz>

fs/btrfs/volumes.c

@@ -5262,7 +5262,6 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info, int rw,
 	    need_raid_map && ((rw & (REQ_WRITE | REQ_GET_READ_MIRRORS)) ||
 	    mirror_num > 1)) {
 		u64 tmp;
-		int i;
 		unsigned rot;
 
 		bbio->raid_map = (u64 *)((void *)bbio->stripes +
@@ -5438,9 +5437,7 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info, int rw,
 			}
 		}
 		if (found) {
-			u64 length = map->stripe_len;
-
-			if (physical_of_found + length <=
+			if (physical_of_found + map->stripe_len <=
 			    dev_replace->cursor_left) {
 				struct btrfs_bio_stripe *tgtdev_stripe =
 					bbio->stripes + num_stripes;