Since we can only hold one shared spinlock at a time anyway, change the

Since we can only hold one shared spinlock at a time anyway, change the
gd_spinlocks_rd counter into a gd_spinlock_rd pointer.  This will improve
performance for potentially contested exclusive spinlocks.  Now they can
test the per-cpu spinlock pointer directly against the spinlock being
acquired instead of testing a counter which might represent any shared
spinlock.

This also has the effect of relaxing the requirement that further
exclusive spinlocks cannot be acquired while holding a shared spinlock,
but for now we are going to leave the requirement intact.