urcu: use unsigned long instead of long

As it is currently designed, urcu scheme never overflows and we do not use the
upper "sign" bit. But, nevertheless, just in case we choose to do it differently
in the future, use an unsigned long type to allow:

- overflow
- defined used of 0x80000000

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>