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

(cherry picked from commit 68f08e26e2797707c76f8c6423baa41b25dd9df4)