vfs: add vfs_tmpfile_open() helper

This helper unifies tmpfile creation with opening.

Existing vfs_tmpfile() callers outside of fs/namei.c will be converted to
using this helper.  There are two such callers: cachefile and overlayfs.

The cachefiles code currently uses the open_with_fake_path() helper to open
the tmpfile, presumably to disable accounting of the open file.  Overlayfs
uses tmpfile for copy_up, which means these struct file instances will be
short lived, hence it doesn't really matter if they are accounted or not.
Disable accounting in this helper too, which should be okay for both
callers.

Add MAY_OPEN permission checking for consistency.  Like for create(2)
read/write permissions are not checked.

Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>