fs: add Jan Kara as reviewer

Jan's been really essential in help deal with reviews in a bunch of
areas and we should really make him an official reviewer. This is long
overdue imho.

Link: https://lore.kernel.org/r/20231205-aufkam-neukunden-d14970a0a6cc@braunerSigned-off-by: Christian Brauner <brauner@kernel.org>

MAINTAINERS

@@ -8157,6 +8157,7 @@ F:	include/trace/events/fs_dax.h
 FILESYSTEMS (VFS and infrastructure)
 M:	Alexander Viro <viro@zeniv.linux.org.uk>
 M:	Christian Brauner <brauner@kernel.org>
+R:	Jan Kara <jack@suse.cz>
 L:	linux-fsdevel@vger.kernel.org
 S:	Maintained
 F:	fs/*