qsbr: portability fixes: use unsigned long for the gp counters.

qsbr: portability fixes: use unsigned long for the gp counters.

unsigned wraps properly in C (modulo ULONG_MAX + 1), IOW ULONG_MAX + 2
_is_ defined and is 1.

Rewrite rcu_gp_ongoing test to work in the unsigned case:

    a - b < 0

becomes

    a - b > ULONG_MAX / 2

This test actually means a - b > 0x7f....f which is exactly what we want
to test for in the first place, but in a portable way.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>