btrfs: make sure to initialize start and len in find_free_dev_extent

Jens reported a compiler error when using CONFIG_CC_OPTIMIZE_FOR_SIZE=y
that looks like this

  In function ‘gather_device_info’,
      inlined from ‘btrfs_create_chunk’ at fs/btrfs/volumes.c:5507:8:
  fs/btrfs/volumes.c:5245:48: warning: ‘dev_offset’ may be used uninitialized [-Wmaybe-uninitialized]
   5245 |                 devices_info[ndevs].dev_offset = dev_offset;
	|                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
  fs/btrfs/volumes.c: In function ‘btrfs_create_chunk’:
  fs/btrfs/volumes.c:5196:13: note: ‘dev_offset’ was declared here
   5196 |         u64 dev_offset;

This occurs because find_free_dev_extent is responsible for setting
dev_offset, however if we get an -ENOMEM at the top of the function
we'll return without setting the value.

This isn't actually a problem because we will see the -ENOMEM in
gather_device_info() and return and not use the uninitialized value,
however we also just don't want the compiler warning so rework the code
slightly in find_free_dev_extent() to make sure it's always setting
*start and *len to avoid the compiler warning.
Reported-by: Jens Axboe <axboe@kernel.dk>
Tested-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -1594,7 +1594,7 @@ static int find_free_dev_extent(struct btrfs_device *device, u64 num_bytes,
 	u64 search_start;
 	u64 hole_size;
 	u64 max_hole_start;
-	u64 max_hole_size;
+	u64 max_hole_size = 0;
 	u64 extent_end;
 	u64 search_end = device->total_bytes;
 	int ret;
@@ -1602,17 +1602,16 @@ static int find_free_dev_extent(struct btrfs_device *device, u64 num_bytes,
 	struct extent_buffer *l;
 
 	search_start = dev_extent_search_start(device);
+	max_hole_start = search_start;
 
 	WARN_ON(device->zone_info &&
 		!IS_ALIGNED(num_bytes, device->zone_info->zone_size));
 
 	path = btrfs_alloc_path();
-	if (!path)
-		return -ENOMEM;
-
-	max_hole_start = search_start;
-	max_hole_size = 0;
-
+	if (!path) {
+		ret = -ENOMEM;
+		goto out;
+	}
 again:
 	if (search_start >= search_end ||
 		test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state)) {