losetup: avoid infinite busy loop

losetup: avoid infinite busy loop

issue report:
 if i run the heavy duty test from #16859 a couple of times I can get
 the loopback layer in the kernel into a state where there's a loopback
 block device allocated, that you can open, but where both LOOP_CLR_FD
 and _SET_FD fail with EBUSY. and /dev/loop-control still returns it as
 the next free one...  weird state util-linux losetup when called to
 allocate a new device then freezes

This commit:
* restrict number of attempts to 16
* use 200000ms sleep between attempts
* add note about non-atomic loop device setup to the man page

Reported-by: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Karel Zak <kzak@redhat.com>