linuxkpi: Add down_write_nest_lock

Simply calls down_write like in Linux (when CONFIG_DEBUG_LOCK_ALLOC isn't
specified)
Needed by drm v5.10

MFC after: 1 month
Reviewed by: hselasky
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D34642