linuxkpi: Implement atomic_dec_and_lock_irqsave()

This is needed by the drm-kmod 5.5 update.

Reviewed by: hselasky, manu
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29988

(cherry picked from commit ce65353ac1a17677bef03e96df8da967c9086743)