random: always update the entropy pool under the spinlock

Instead of using lockless techniques introduced in commit
902c098a3663, use spin_trylock to try to grab entropy pool's lock.  If
we can't get the lock, then just try again on the next interrupt.

Based on discussions with George Spelvin.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: George Spelvin <linux@horizon.com>