MFC r327697, r327699:

Use the __result_use_check attribute for reallocf(9).

MFC r327751:
Use the __result_use_check attribute also for reallocf(3).

The GCC attribute causes a warning to be emitted if a caller of the
function with this attribute does not use its return value. Unlike the
traditional realloc, with reallocf(3) we don't have to check for NULL
values but we still have to make sure the result is used.