Without this, somebody trying to map a buffer for write by the CPU would
fail. This is not common to do in hardware driver environments, but it
shouldn't be disallowed, and there's no downside to allowing it.
I did skip virgl, because that's one where I don't know for sure if there
wouldn't be a downside to allowing RDWR (there are other virt exports
where RDWR is gated on a mappable flag).
This is a follow-up to !37088 to keep copy and paste from introducing the
same bug anywhere else.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37339>
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