seqlock,tags: Add support for SEQCOUNT_LOCKTYPE()

Such that we might easily find seqcount_LOCKTYPE_t and
seqcount_LOCKTYPE_init().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200729161938.GB2678@hirez.programming.kicks-ass.net