kboot: Convert from getdents to getdents64

Linux 2.4 introduced getdents64. Switch to using it because aarch64
doesn't have getdents as that syscall was obsoleted before that port was
created.

Sponsored by: Netflix

(cherry picked from commit 3a1b966ae0c1b9f334e1e3a7ad8b4bf8e845f3c5)