vfio: selftests: Verify a failed second open preserves the vf_token

The cdev path enforces a single open per device and rejects a second
bind of an already open device.  That rejection must happen before the
bind can mutate state shared across opens, notably the PF vf_token, so
that a bind which cannot complete leaves the current opener's state
untouched.  Before the fix in commit 258ba46543ab ("vfio: Reject a
second cdev open before mutating shared device state"), the second bind
mutated the vf_token and only then failed with -EINVAL.  It is now
rejected early with -EBUSY.

Add a regression test that binds a PF with one token, attempts a second
bind of the same PF with a different token, then initializes a VF with
the original token.  The VF init succeeds only if the rejected second
bind left the PF vf_token intact; a regression that clobbered it to the
second token would make the VF init fail.  The -EBUSY errno is checked
with EXPECT_EQ() so the clobber assertion still runs if the rejection
returns a different error.

This hazard is specific to the cdev/iommufd single-open path, so the
test runs only in iommufd mode.

Suggested-by: David Matlack <dmatlack@google.com>
Assisted-by: LLM
Signed-off-by: Alex Williamson <alex.williamson@nvidia.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20260925205134.3505611-1-alex.williamson@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

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