[iobuf] Limit automatic I/O buffer alignment to page size

[iobuf] Limit automatic I/O buffer alignment to page size

Without any explicit alignment requirement, we will currently allocate
I/O buffers on their own size rounded up to the nearest power of two.
This is done to simplify driver transmit code paths, which can assume
that a standard Ethernet frame lies within a single physical page and
therefore does not need to be split even for devices with DMA engines
that cannot cross page boundaries.

Limit this automatic alignment to a maximum of the page size, to avoid
requiring excessive alignment for unusually large buffers (such as a
buffer allocated for an HTTP POST with a large parameter list).

Signed-off-by: Michael Brown <mcb30@ipxe.org>