seqlock: Rename __seqprop() users

More consistent naming should make it easier to untangle the _Generic
token pasting maze called __seqprop().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201110115358.GE2594@hirez.programming.kicks-ass.net