Make __rand_0_1() compatible with clang

Make __rand_0_1() compatible with clang

This patch fixes the following clang compiler error:

crc/../arch/../lib/rand.h:109:25: error: implicit conversion from
      'unsigned long long' to 'double' changes value from 18446744073709551615
      to 18446744073709551616 [-Werror,-Wimplicit-int-float-conversion]
                return (val + 1.0) / (FRAND64_MAX + 1.0);
                                      ^~~~~~~~~~~ ~
crc/../arch/../lib/rand.h:9:22: note: expanded from macro 'FRAND64_MAX'
 #define FRAND64_MAX     (-1ULL)
                         ^~~~~

Fixes: e7b240474543 ("Fixups for poisson rate")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>