posix: build the io pool from the clamped size, not the raw request (#1827)

nixlPosixIOQueueImpl sized its ios_ pool and free_ios_ free-list from the raw ios_pool_size constructor argument instead of the clamped result in ios_pool_size_. This PR builds the pool from ios_pool_size_ so the pool and the rest of the class agree.

Signed-off-by: Eylon Krause <eylon1909@gmail.com>