LinuxKPI: Allow spin_lock_irqsave to be called within a critical section

with spinning on spin_trylock. dma-buf part of drm-kmod depends on this
property and absence of it support results in "mi_switch: switch in a
critical section" assertions [1][2].

[1] https://github.com/freebsd/drm-kmod/issues/116
[2] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261166

MFC after: 1 week
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D33887