rseq: Optimise rseq_get_rseq_cs() and clear_rseq_cs()

Commit ec9c82e0

 ("rseq: uapi: Declare rseq_cs field as union,
update includes") added regressions for our servers.

Using copy_from_user() and clear_user() for 64bit values
is suboptimal.

We can use faster put_user() and get_user() on 64bit arches.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lkml.kernel.org/r/20210413203352.71350-4-eric.dumazet@gmail.com