MAINTAINERS: add coverage for drivers/block

To help folks like me that use scripts/get_maintainer.pl.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

MAINTAINERS

@@ -2683,6 +2683,7 @@ L:	linux-block@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git
 S:	Maintained
 F:	block/
+F:	drivers/block/
 F:	kernel/trace/blktrace.c
 F:	lib/sbitmap.c