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>
diff --git a/fs/fat/Kconfig b/fs/fat/Kconfig
index 238cc55..afe83b4 100644
--- a/fs/fat/Kconfig
+++ b/fs/fat/Kconfig
@@ -2,6 +2,7 @@
 config FAT_FS
 	tristate
 	select NLS
+	select LEGACY_DIRECT_IO
 	help
 	  If you want to use one of the FAT-based file systems (the MS-DOS and
 	  VFAT (Windows 95) file systems), then you must say Y or M here