storage_file_probe: Remove BACKING_STORE_OK,BACKING_STORE_INVALID states

storage_file_probe: Remove BACKING_STORE_OK,BACKING_STORE_INVALID states

Replace the return values by 0 because none of the callers care and some
of the backing store parser functions return this state also in cases
the rest of the code would consider as success.

Subsequently the parsers will be refactored and proper error reporting
returned.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>