rcu/nocb: Suppress uninitialized false-positive in nocb_gp_wait()

Some compilers complain that wait_gp_seq might be used uninitialized
in nocb_gp_wait().  This cannot actually happen because when wait_gp_seq
is uninitialized, needwait_gp must be false, which prevents wait_gp_seq
from being used.  But this analysis is apparently beyond some compilers,
so this commit adds a bogus initialization of wait_gp_seq for the sole
purpose of suppressing the false-positive warning.

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