random: fix typo in add_timer_randomness()

s/entimate/estimate

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Link: https://lore.kernel.org/r/20190607182517.28266-4-tiny.windzz@gmail.com


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