virtiofsd: Don't allow file creation with FUSE_OPEN

A well behaved FUSE client uses FUSE_CREATE to create files. It isn't
supposed to pass O_CREAT along a FUSE_OPEN request, as documented in
the "fuse_lowlevel.h" header :

    /**
     * Open a file
     *
     * Open flags are available in fi->flags. The following rules
     * apply.
     *
     *  - Creation (O_CREAT, O_EXCL, O_NOCTTY) flags will be
     *    filtered out / handled by the kernel.

But if the client happens to do it anyway, the server ends up passing
this flag to open() without the mandatory mode_t 4th argument. Since
open() is a variadic function, glibc will happily pass whatever it
finds on the stack to the syscall. If this file is compiled with
-D_FORTIFY_SOURCE=2, glibc will even detect that and abort:

*** invalid openat64 call: O_CREAT or O_TMPFILE without mode ***: terminated

Specifying O_CREAT with FUSE_OPEN is a protocol violation. Check this
in do_open(), print out a message and return an error to the client,
EINVAL like we already do when fuse_mbuf_iter_advance() fails.

The FUSE filesystem doesn't currently support O_TMPFILE, but the very
same would happen if O_TMPFILE was passed in a FUSE_OPEN request. Check
that as well.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20210624101809.48032-1-groug@kaod.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>