Revert "cred: switch to using atomic_long_t"

This reverts commit 123e44b9 which is
commit f8fa5d76925991976b3e7076f9d1052515ec1fca upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way.

Bug: 161946584
Bug: 317347552
Change-Id: I9eb52866bc3b2a9f02e3f7fbee0bacbcc06b0849
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>