ovl: implement tmpfile

Combine inode creation with opening a file.

There are six separate objects that are being set up: the backing inode,
dentry and file, and the overlay inode, dentry and file.  Cleanup in case
of an error is a bit of a challenge and is difficult to test, so careful
review is needed.

All tmpfile testcases except generic/509 now run/pass, and no regressions
are observed with full xfstests.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
diff --git a/fs/internal.h b/fs/internal.h
index 7ca7389..ab22251 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -62,6 +62,9 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode);
 int do_symlinkat(struct filename *from, int newdfd, struct filename *to);
 int do_linkat(int olddfd, struct filename *old, int newdfd,
 			struct filename *new, int flags);
+int vfs_tmpfile(struct mnt_idmap *idmap,
+		const struct path *parentpath,
+		struct file *file, umode_t mode);
 
 /*
  * namespace.c