drm/i915/selftests: Keep mock file open during unfaultable migrate with fill

drm/i915/selftests: Keep mock file open during unfaultable migrate with fill

igt_mmap_migrate() tests migration with various parameters.
In one of the cases, where FILL and UNFAULTABLE flags are set,
during first stages of this test, a mock file is opened in
igt_mmap_offset(), which results in allocating GEM objects for
page table structures and scratch in GPU mappable memory.

Then, also in igt_mmap_offset(), the file is closed (fput) and
the cleanup of these objects is scheduled on a delayed worqueue,
which is designed to execute after unspecified amount of time.

Next, the test calls igt_fill_mappable() to fill mappable GPU
memory. At this point, three scenarios are possible
(N = max size of GPU memory for this test in MiB):
 1) the objects allocated for the mock file get cleaned up after
    crucial part of the test is over, so the memory is full with
    the 1 MiB they occupy and N - 1 MiB added by
    igt_fill_mappable(), so the migration fails properly;
 2) the object cleanup fires before igt_fill_mappable()
    completes, so the whole memory is populated with N MiB from
    igt_fill_mappable(), so migration fails as well;
 3) the object cleanup is performed right after fill is done,
    so only N - 1 MiB are in the mappable portion of GPU memory,
    allowing the migration to succeed - we'd expect no space
    left to perform migration, but an object was able to fit in
    the remaining 1 MiB, which caused get_user() to succeed, so
    a page fault did not fail.

The test incorrectly assumes that the GPU mappable memory state
is unchanging during the test. Amend this by keeping the mock
file open until migration and page fault checking is complete.

Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/13929
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Acked-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/6xc74s3mbmtliqxihtxbok32jobhc26vfm5mu6cod4ywov6utf@ujp3rmqcwmr3
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>