Tautological Compare warning fixes

Since unsigned integers can never be negative, some code can never be
reached and can safely be removed as dead code.

The underlying function zvolIO_kit_read/zvolIO_kit_write is using IOMemoryDescriptor::writeBytes/IOMemoryDescriptor::readBytes respective, which also have an unsigned return value. It is also documented to never return a negative number.

https://developer.apple.com/library/mac/documentation/Kernel/Reference/IOMemoryDescriptor_reference/translated_content/IOMemoryDescriptor.html#//apple_ref/cpp/instm/IOMemoryDescriptor/writeBytes/IOByteCount