linux(4): Fixup waitid handling P_PGID idtype.

Since Linux 5.4, if id is zero, then wait for any child that is in the same
process grop as the caller's process group.

Differential revision: https://reviews.freebsd.org/D31567
MFC after: 2 weeks

(cherry picked from commit 4e3aefb923ac1bd8e2a707b00239f171eab1231b)