virtio-net: avoid memset() on intermediate 64k buffer for each TX packet
virtio-net: avoid memset() on intermediate 64k buffer for each TX packet

Intermediate 64k buffer used to build the frame being TX'ed by the guest
was being initialized to all zeroes for each TX packet, thus wasting
a lot of CPU cycles.
Avoid doing that by keeping the buffer in heap instead of on the stack.

Signed-off-by: Adrian Catangiu <43487eb7d05f4ed2cf28dd3e748821285e5f7796@amazon.com>

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