btrfs: remove set but not used variable 'offset'

Fixes gcc '-Wunused-but-set-variable' warning:

fs/btrfs/volumes.c: In function __btrfs_map_block:
fs/btrfs/volumes.c:6023:6: warning:
 variable offset set but not used [-Wunused-but-set-variable]

It is not used any more since commit 343abd1c0ca9 ("btrfs: Use
btrfs_get_io_geometry appropriately")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6011,7 +6011,6 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
 {
 	struct extent_map *em;
 	struct map_lookup *map;
-	u64 offset;
 	u64 stripe_offset;
 	u64 stripe_nr;
 	u64 stripe_len;
@@ -6046,7 +6045,6 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
 	map = em->map_lookup;
 
 	*length = geom.len;
-	offset = geom.offset;
 	stripe_len = geom.stripe_len;
 	stripe_nr = geom.stripe_nr;
 	stripe_offset = geom.stripe_offset;