Revert "pid: Replace struct pid 1-element array with flex-array"

This reverts commit 5ea9dcfc which is
commit b69f0aeb upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Id78fddc05ab98cf79beff0adc409fbc588f7499f
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>