Modify lock_delay() to increase the delay time after spinning

Modify lock_delay() to increase the delay time after spinning,
not before. Previously we would spin at least twice instead of once.
In NetApp's benchmarks this fixes a performance regression compared
to FreeBSD 10, which called cpu_spinwait() directly.

Reviewed By: mjg
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27331