Reapply "cred: switch to using atomic_long_t"

This reverts commit b2ea47a7 as it is
needed in Android systems.  It still breaks the ABI, but that is no
longer an issue on this branch.

Bug: 317347552
Change-Id: I9c7f8901c5aabf6e0378b5218f587d6c7335522f
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>