exfat: fs: add CONFIG_BUFFER_HEAD

Add a new config option that controls building the buffer_head code, and
select it from all file systems and stacking drivers that need it.

For the block device nodes and alternative iomap based buffered I/O path
is provided when buffer_head support is not enabled, and iomap needs a
a small tweak to define the IOMAP_F_BUFFER_HEAD flag to 0 to not call
into the buffer_head code when it doesn't exist.

Otherwise this is just Kconfig and ifdef changes.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Luis Chamberlain <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Namjae Jeon <[email protected]>

Kconfig

@@ -2,6 +2,7 @@
 
 config EXFAT_FS
 	tristate "exFAT filesystem support"
+	select BUFFER_HEAD
 	select NLS
 	select LEGACY_DIRECT_IO
 	help