udp: not remove the CRC flag from dev features when need_csum is false

In __skb_udp_tunnel_segment(), when it's a SCTP over VxLAN/GENEVE
packet and need_csum is false, which means the outer udp checksum
doesn't need to be computed, csum_start and csum_offset could be
used by the inner SCTP CRC CSUM for SCTP HW CRC offload.

So this patch is to not remove the CRC flag from dev features when
need_csum is false.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/1e81b700642498546eaa3f298e023fd7ad394f85.1610776757.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>