srcu_tree: Disable preemption across synchronize_srcu_atomic()

Because synchronize_srcu_atomic() cannot sleep, if a pair of them run
concurrently pinned to the same CPU, it is possible that one will spin
uselessly waiting for the other while at the same time preventing that
other from running.  This commit therefore disables preemption to prevent
this failure mode.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>