random: remove unnecessary unlikely()

WARN_ON() already contains an unlikely(), so it's not necessary to use
unlikely.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Link: https://lore.kernel.org/r/20190607182517.28266-1-tiny.windzz@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>