There is no reason for the chacha20poly1305 SG miter code to use
kmap instead of kmap_atomic as the critical section doesn't sleep
anyway.  So we can simply get rid of the preemptible check and
set SG_MITER_ATOMIC unconditionally.

Even if we need to reenable preemption to lower latency we should
be doing that by interrupting the SG miter walk rather than using
kmap.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>