Replace vfork() with fork().

The static analyzer complained about the function calls we were doing
here to vector::data() and vector::operator[]:
bootable/recovery/roots.cpp:193:11: warning: This function call is
prohibited after a successful vfork

Since it's not clear that vfork() is needed here, just use fork()
instead.

Bug: None
Test: Reran the static analyzer with only this patch applied. Bug
disappeared
Change-Id: I580f8243a21899f1e1678c8aee6948dfa7f69a1d