MAINTAINERS: add block and fsdevel lists to iov_iter

We've had multiple instances where people didn't Cc fsdevel or block
which are easily the most affected subsystems by iov_iter changes.
Put a stop to that and make sure both lists are Cced so we can catch
stuff like [1] early.

Link: https://lore.kernel.org/linux-nvme/20250715132750.9619-4-aaptel@nvidia.com [1]
Link: https://lore.kernel.org/20250716-eklig-rasten-ec8c4dc05a1e@brauner
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

MAINTAINERS

@@ -25907,6 +25907,8 @@ F:	fs/hostfs/
 
 USERSPACE COPYIN/COPYOUT (UIOVEC)
 M:	Alexander Viro <viro@zeniv.linux.org.uk>
+L:	linux-block@vger.kernel.org
+L:	linux-fsdevel@vger.kernel.org
 S:	Maintained
 F:	include/linux/uio.h
 F:	lib/iov_iter.c