rcu: Convert ->rcu_iw_gpnum to ->gp_seq

This commit switches the interrupt-disabled detection mechanism to
->gp_seq.  This mechanism is used as part of RCU CPU stall warnings,
and detects cases where the stall is due to a CPU having interrupts
disabled.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>