btrfs: scrub: pass fs_info to scrub_setup_ctx

[ Upstream commit 92f7ba434f51e8e9317f1d166105889aa230abd2 ]

We can pass fs_info directly as this is the only member of btrfs_device
that's bing used inside scrub_setup_ctx.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>