drm/i915: Prevent a race during I915_GEM_MMAP ioctl with WC set

Make sure the underlying VMA in the process address space is the
same as it was during vm_mmap to avoid applying WC to wrong VMA.

A more long-term solution would be to have vm_mmap_locked variant
in linux/mmap.h for when caller wants to hold mmap_sem for an
extended duration.

v2:
- Refactor the compare function

Fixes: 1816f9236303 ("drm/i915: Support creation of unbound wc user mappings for objects")
Reported-by: Adam Zabrocki <[email protected]>
Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Joonas Lahtinen <[email protected]>
Cc: <[email protected]> # v4.0+
Cc: Akash Goel <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Adam Zabrocki <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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