Fix getdirentries(2) under 32-bit compat.

The latest version of getdirentries (syscall 554) takes a pointer
an an off_t as the last argument. The old version which copies out
an int32_t was being used instead. Use the standard sys_getdirentries()
implementation instead.

Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14379