loop-util: LOOP_CLR_FD is async, don't retry to reuse a device right after issuing it

loop-util: LOOP_CLR_FD is async, don't retry to reuse a device right after issuing it

When we fall back to classic LOOP_SET_FD logic in case LOOP_CONFIGURE
didn't work we issue LOOP_CLR_FD first. But that call turns out to be
potentially async in the kernel: if something else (let's say
udev/blkid) is accessing the device the ioctl just sets the autoclear
flag and exits. Hence quite often the LOOP_SET_FD will subsequently
fail. Let's avoid the trouble, and immediately exit with EBUSY if
LOOP_CONFIGURE fails, and but remember that LOOP_CONFIGURE is not
available so that on the next iteration we go directly for LOOP_SET_FD
instead.