shared/loop-util: fix error handling in loop_device_make_full()

The function no longer returns the fd. This complicated semantics, because it
wasn't clear what holds the ownership: the return value or the output
parameter.  There were no users of the fd in the return value, so let's
simplify things conceptually and only return the fd once.

Reduce the scope of variables.

LOOP_CLR_FD was called on the wrong fd. Let's use a cleanup function to make
this automatic and reduce chances of a mixup in the future.

CID 1408498.