LinuxKPI: Add write_seqcount_invalidate() and seqprop_sequence()

functions to linux/seqlock.h

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42810

(cherry picked from commit 64e30cba3f0432fa684728b67850b6254d3713ea)