UPSTREAM: rcu: Remove __read_mostly annotations from rcu_scheduler_active externs

Remove the __read_mostly attributes from the rcu_scheduler_active
extern declarations, because these attributes are ignored for
prototypes and we'd have to include the full <linux/cache.h> header
to gain this functionally pointless attribute defined.

BUG: 274028827
Change-Id: Ib9c1d34ad2325b8cf939f5df0b35ebe4b281e9ec
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
(cherry picked from commit e6339d3b)
Signed-off-by: Srinivasarao Pathipati <quic_spathi@quicinc.com>