lmkd: Track processes using pidfds

lmkd uses PIDs to track processes, however occasionally a PID of a process
might be reused without lmkd detecting that. This can happen if originally
registered process crashes, PID numbers wrap around and the same PID gets
reused for a different process. In this situation lmkd might kill a wrong
process. To prevent this issue from occurring lmkd will track processes
using their pidfd. During process registration lmkd calls sys_pidfd_open
and stores returned pidfd with the process record. Returned pidfd will not
be reused until lmkd closes it which happens only after the process is
unregistered. This way lmkd ensures that process identification is unique
and can't be reused.

Bug: 135608568
Test: lmkd_unit_test with and without pidfd kernel support
Change-Id: Ida10ea13905c250e47f792cdd6bd2e65aeaa3709
Signed-off-by: Suren Baghdasaryan <surenb@google.com>