s390x: Mark archrandom.h functions __must_check

We must not use the pointer output without validating the
success of the random read.

Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200110145422.49141-11-broonie@kernel.org


Signed-off-by: Theodore Ts'o <tytso@mit.edu>