EC: Excess internal block may not be able to be deleted correctly when it's stored in fallback storage

Excess internal block can't be deleted correctly when it's stored in fallback storage.

Simple case:

EC-RS-6-3-1024k file is stored using ALL_SSD storage policy(SSD is default storage type and DISK is fallback storage type), if the block group is as follows

[0(SSD), 0(SSD), 1(SSD), 2(SSD), 3(SSD), 4(SSD), 5(SSD), 6(SSD), 7(SSD), 8(DISK)] 

The are two index 0 internal block and one of them should be chosen to delete.But the current implement chooses the index 0 internal blocks as candidates but DISK as exess storage type.As a result, the exess storage type(DISK) can not correspond to the exess internal blocks' storage type(SSD) correctly, and the exess internal block can not be deleted correctly.