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: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>