loop-util: wait a random time before trying again

loop-util: wait a random time before trying again

Let's try to make collisions when multiple clients want to use the same
device less likely, by sleeping a random time on collision.

The loop device allocation protocol is inherently collision prone:
first, a program asks which is the next free loop device, then it tries
to acquire it, in a separate, unsynchronized setp. If many peers do this
all at the same time, they'll likely all collide when trying to
acquire the device, so that they need to ask for a free device again and
again.

Let's make this a little less prone to collisions, reducing the number
of failing attempts: whenever we notice a collision we'll now wait
short and randomized time, making it more likely another peer succeeds.

(This also adds a similar logic when retrying LOOP_SET_STATUS64, but
with a slightly altered calculation, since there we definitely want to
wait a bit, under all cases)