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/timerfd.c b/fs/timerfd.c
index b042bd7..1bfc95a 100644
--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -200,7 +200,7 @@
 	hrtimer_init(&ctx->tmr, clockid, HRTIMER_MODE_ABS);
 
 	ufd = anon_inode_getfd("[timerfd]", &timerfd_fops, ctx,
-			       flags & TFD_SHARED_FCNTL_FLAGS);
+			       O_RDWR | (flags & TFD_SHARED_FCNTL_FLAGS));
 	if (ufd < 0)
 		kfree(ctx);