Implement big pool tracker table expansion

When using a large number of big pool allocations, the pool tracker table will run out of space, leading to the print

Big pool expansion needed, not implemented!

When these allocations then get freed, ExpFindAndRemoveTagBigPages returns ' GIB' because it did not find an appropriate table entry. This can lead to BAD_POOL_CALLER bugchecks.

Btrfs seems to run into this issue frequently due to its high pool usage.