ixgbevf: improve performance and reduce size of ixgbevf_tx_map()

ixgbevf: improve performance and reduce size of ixgbevf_tx_map()

Based on commit ec718254cbfe
("ixgbe: Improve performance and reduce size of ixgbe_tx_map")

This change is meant to both improve the performance and reduce the size of
ixgbevf_tx_map().

Expand the work done in the main loop by pushing first into tx_buffer.
This allows us to pull in the dma_mapping_error check, the tx_buffer value
assignment, and the initial DMA value assignment to the Tx descriptor.

Signed-off-by: Emil Tantilov <[email protected]>
Tested-by: Krishneil Singh <[email protected]>
Signed-off-by: Jeff Kirsher <[email protected]>