This is basically equivalent to the FUSE_CREATE operation which creates and
opens a regular file.

Add a new FUSE_TMPFILE operation, otherwise just reuse the protocol and the
code for FUSE_CREATE.
Acked-by: NChristian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>