ethdev: add new offload flag to keep CRC

ethdev: add new offload flag to keep CRC

DEV_RX_OFFLOAD_KEEP_CRC offload flag is added. PMDs that support
keeping CRC should advertise this offload capability.

DEV_RX_OFFLOAD_CRC_STRIP flag will remain one more release
default behavior in PMDs are to keep the CRC until this flag removed

Until DEV_RX_OFFLOAD_CRC_STRIP flag is removed:
- Setting both KEEP_CRC & CRC_STRIP is INVALID
- Setting only CRC_STRIP PMD should strip the CRC
- Setting only KEEP_CRC PMD should keep the CRC
- Not setting both PMD should keep the CRC

A helper function rte_eth_dev_is_keep_crc() has been added to be able to
change the no flag behavior with minimal changes in PMDs.

The PMDs that doesn't report the DEV_RX_OFFLOAD_KEEP_CRC offload can
remove rte_eth_dev_is_keep_crc() checks next release, related code
commented to help the maintenance task.

And DEV_RX_OFFLOAD_CRC_STRIP has been added to virtual drivers since
they don't use CRC at all, when an application requires this offload
virtual PMDs should not return error.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Allain Legacy <allain.legacy@windriver.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>