srcu: Make srcu_funnel_gp_start() cache ->mynode in snp_leaf

Currently, the srcu_funnel_gp_start() walks its local variable snp up the
tree and reloads sdp->mynode whenever it is necessary to check whether
it is still at the leaf srcu_node level.  This works, but is a bit more
obtuse than absolutely necessary.  In addition, upcoming commits will
dynamically size srcu_struct structures, in which case sdp->mynode will
no longer necessarily be a constant, and this commit helps prepare for
that dynamic sizing.

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