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/[email protected] [1]
Link: https://lore.kernel.org/20250716-eklig-rasten-ec8c4dc05a1e@brauner
Reviewed-by: Jens Axboe <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: brauner

MAINTAINERS

@@ -25907,6 +25907,8 @@ F:	fs/hostfs/
 
 USERSPACE COPYIN/COPYOUT (UIOVEC)
 M:	Alexander Viro <[email protected]>
+L:	[email protected]
+L:	[email protected]
 S:	Maintained
 F:	include/linux/uio.h
 F:	lib/iov_iter.c