[hack] Correct usage of posix_fallocate
[hack] Correct usage of posix_fallocate

Summary:
posix_fallocate takes an offset, not a pointer. The bug that the
previous diff tried to fix was that `memfd_reserve` was being called in the
first place even though the memory that we were trying to reserve was not
actually in memfd at all.

Reviewed By: andrewjkennedy, dlreeves

Differential Revision: D3623164

fbshipit-source-id: 9feb491faa865d3e14f07df8cb674ec5916faabd

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