btrfs: Set io_tree only once in submit_compressed_extents

The inode never changes so it's sufficient to dereference it and get
the iotree only once, before the execution of the main loop. No
functional changes, only the size of the function is decreased:

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-44 (-44)
Function                                     old     new   delta
submit_compressed_extents                   1240    1196     -44
Total: Before=88476, After=88432, chg -0.05%
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -717,7 +717,7 @@ static noinline void submit_compressed_extents(struct async_chunk *async_chunk)
 	struct btrfs_key ins;
 	struct extent_map *em;
 	struct btrfs_root *root = BTRFS_I(inode)->root;
-	struct extent_io_tree *io_tree;
+	struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
 	int ret = 0;
 
 again:
@@ -725,9 +725,6 @@ static noinline void submit_compressed_extents(struct async_chunk *async_chunk)
 		async_extent = list_entry(async_chunk->extents.next,
 					  struct async_extent, list);
 		list_del(&async_extent->list);
-
-		io_tree = &BTRFS_I(inode)->io_tree;
-
 retry:
 		/* did the compression code fall back to uncompressed IO? */
 		if (!async_extent->pages) {