sys/mutex.h: Include sys/lock.h instead of sys/_lock.h
It uses the LA_ defines when INVARIANTS is set.
This unbreak dpaa2 with FDT only kernel (like ALLWINNER or ROCKCHIP) as
the driver only include sys/lock.h via header polution for ACPI kernels.
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D37145
Reviewed by: kib, mjg