linuxkpi: Add seqcount_ww_mutex_t in <linux/seqlock.h>

It is defined as a synonymous to seqcount_mutex_t.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38152

(cherry picked from commit b34cd67243c39ca55be40b3e7958f3ade290347c)