MAINTAINERS: add Brian Foster as a reviewer for bcachefs

Brian has been playing with bcachefs for several months now and has
offerred to commit time to patch review.
Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

MAINTAINERS

@@ -3466,6 +3466,7 @@ F:	drivers/md/bcache/
 
 BCACHEFS
 M:	Kent Overstreet <kent.overstreet@linux.dev>
+R:	Brian Foster <bfoster@redhat.com>
 L:	linux-bcachefs@vger.kernel.org
 S:	Supported
 C:	irc://irc.oftc.net/bcache