fix "This _Backstore already exists in configFS" exception
fix "This _Backstore already exists in configFS" exception

Creating multiple backstores at the same time without specifying the
index parameter may cause this exception.
The reason is that _Backstore is allocated the same new index value.

Signed-off-by: b4ff817490cf35c938d84ff398331095c2234b17@easystack.cn

cvubrugier/rtslib-fb