vmxnet3: Provide required number of bytes in first SG buffer

This is a performance enhancement fix. vmxnet3 device performs better when
provided with at least 54 bytes (ethernet 14 + IP 20+ TCP 20) in the first SG
buffer. For UDP packets driver provides lesser than that in first sg. This
change fixes the same. Also avoid the redundant pskb_may_pull() call.

Signed-off-by: Shreyas N Bhatewara <sbhatewara@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>