LinuxKPI: linux/seqlock.h: Fix write_seqcount_(begin|end)

in seqcount_mutex_t case with removal of extraneous mutex lock/unlock
calls and addition of missing critical section.
While here strip one inline wrap layer to reduce code size.

Fixes startup lockup of i915kms after update to drm-kmod v5.12

Reviewed by: hselasky, bz
MFC after: 1week
Differential Revision: https://reviews.freebsd.org/D37699