aio: mark __aio_sigset::sigmask const

io_pgetevents() will not change the signal mask. Mark it const
to make it clear and to reduce the need for casts in user code.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Avi Kivity <avi@scylladb.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/uapi/linux/aio_abi.h

@@ -109,7 +109,7 @@ struct iocb {
 #undef IFLITTLE
 
 struct __aio_sigset {
-	sigset_t __user	*sigmask;
+	const sigset_t __user	*sigmask;
 	size_t		sigsetsize;
 };