make build_open_flags() treat O_CREAT | O_EXCL as implying O_NOFOLLOW

O_CREAT | O_EXCL means "-EEXIST if we run into a trailing symlink".
As it is, we might or might not have LOOKUP_FOLLOW in op->intent
in that case - that depends upon having O_NOFOLLOW in open flags.
It doesn't matter, since we won't be checking it in that case -
do_last() bails out earlier.

However, making sure it's not set (i.e. acting as if we had an explicit
O_NOFOLLOW) makes the behaviour more explicit and allows to reorder the
check for O_CREAT | O_EXCL in do_last() with the call of step_into()
immediately following it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/open.c b/fs/open.c
index 0788b37..e5227cd 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1049,8 +1049,10 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 
 	if (flags & O_CREAT) {
 		op->intent |= LOOKUP_CREATE;
-		if (flags & O_EXCL)
+		if (flags & O_EXCL) {
 			op->intent |= LOOKUP_EXCL;
+			flags |= O_NOFOLLOW;
+		}
 	}
 
 	if (flags & O_DIRECTORY)