rcu: Flag lockless access to ->gp_flags with ACCESS_ONCE()

This commit applies ACCESS_ONCE() to an outside-of-lock access to
->gp_flags.  Although it is hard to imagine any sane compiler messing
this particular case up, the documentation benefits are substantial.
Plus the definition of "sane compiler" grows ever looser.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Git-commit: 591c6d1710cd73824057d08eda302cf2a7cfd18a
[kishank@codeaurora.org resolve trivial conflicts]
Signed-off-by: Kishan Kumar <kishank@codeaurora.org>