fs: build the legacy direct I/O code conditionally

Add a new LEGACY_DIRECT_IO config symbol that is only selected by the
file systems that still use the legacy blockdev_direct_IO code, so that
kernels without support for those file systems don't need to build the
code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20230125065839.191256-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>