[efi] Provide guaranteed space in transmitted packets

[efi] Provide guaranteed space in transmitted packets

eIPoIB requires space to expand a transmitted ARP packet.  This
guarantee is met by ensuring that a transmitted packet consists of at
least MAX_LL_HEADER_LEN bytes from the start of the I/O buffer up to
the end of the link-layer header, and at least IOB_ZLEN bytes
thereafter.

Adjust the I/O buffer allocation for SNP transmitted packets to ensure
that this guarantee is met.

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