Convert requested buffers to overdraft buffers when pool size is decreased

As we discussed in FLINK-31610, new buffers can be requested only when "numOfRequestedMemorySegments + numberOfRequestedOverdraftMemorySegments < poolSize + maxOverdraftBuffersPerGate".

Consider such a scenario, the CurrentPoolSize = 5, numOfRequestedMemorySegments = 7, maxOverdraftBuffersPerGate = 2. If numberOfRequestedOverdraftMemorySegments = 0, then 2 buffers can be requested now. 

We should convert numberOfRequestedMemorySegments to numberOfRequestedOverdraftMemorySegments when poolSize is decreased. Further more, we can changes the definition of overdraft buffer from static to dynamic: 

• When numberOfRequestedMemorySegments <= poolSize, all buffers are ordinary buffer
• When numberOfRequestedMemorySegments > poolSize, the `ordinary buffer size = poolSize`, and `the overdraft buffer size = numberOfRequestedMemorySegments - poolSize`

This allows us to remove numberOfRequestedOverdraftMemorySegments, which helps us simplify logic and maintain consistency.