anonfd: Allow making anon files read-only

It seems a couple places such as arch/ia64/kernel/perfmon.c and
drivers/infiniband/core/uverbs_main.c could use anon_inode_getfile()
instead of a private pseudo-fs + alloc_file(), if only there were a way
to get a read-only file.  So provide this by having anon_inode_getfile()
create a read-only file if we pass O_RDONLY in flags.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/signalfd.c b/fs/signalfd.c
index b07565c..1dabe4e 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -236,7 +236,7 @@
 		 * anon_inode_getfd() will install the fd.
 		 */
 		ufd = anon_inode_getfd("[signalfd]", &signalfd_fops, ctx,
-				       flags & (O_CLOEXEC | O_NONBLOCK));
+				       O_RDWR | (flags & (O_CLOEXEC | O_NONBLOCK)));
 		if (ufd < 0)
 			kfree(ctx);
 	} else {