uipc_shm: silent warnings about write-only variables in largepage code

In shm_largepage_phys_populate(), the result from vm_page_grab() is only
needed for assertion.

In shm_dotruncate_largepage(), there is a commented-out prototype code
for managed largepages. The oldobjsz is saved for its sake, so mark
the variable as __unused directly.

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