LinuxKPI: Do not wait for a grace period in rcu_barrier()

Linux docs explicitly state that this is not required [1]:

"Important note: The rcu_barrier() function is not, repeat, not,
obligated to wait for a grace period. It is instead only required to
wait for RCU callbacks that have already been posted. Therefore, if
there are no RCU callbacks posted anywhere in the system, rcu_barrier()
is within its rights to return immediately. Even if there are
callbacks posted, rcu_barrier() does not necessarily need to wait for
a grace period."

[1] https://www.kernel.org/doc/Documentation/RCU/Design/Requirements/Requirements.html

Reviewed by: emaste, hselasky, manu
Differential revision: https://reviews.freebsd.org/D30809

(cherry picked from commit 1ab61a193241f832e63f97ca37880b010469cb38)