block: move CONFIG_BLOCK guard to top Makefile

[ Upstream commit 4c928904

 ]

Every object under block/ depends on CONFIG_BLOCK.

Move the guard to the top Makefile since there is no point to
descend into block/ if CONFIG_BLOCK=n.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210927140000.866249-5-masahiroy@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: 998b30c3

 ("io_uring: Fix a null-ptr-deref in io_tctx_exit_cb()")
Signed-off-by: Sasha Levin <sashal@kernel.org>