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I/O buffers are allocated on aligned boundaries. The I/O buffer descriptor (the struct io_buffer) is currently attached to the end of the I/O buffer. When the size of the buffer is close to its alignment, this can waste large amounts of aligned memory. For example, a network card using 2048-byte receive buffers will end up allocating 2072 bytes on a 2048-byte boundary. This effectively wastes 50% of the available memory. Improve the situation by allocating the descriptor separately from the main I/O buffer if inline allocation would cause the total allocated size to cross the alignment boundary. Signed-off-by: Michael Brown <mcb30@ipxe.org>