if_vxlan: fix byteorder of source port
Fix the htons byteorder of vxlan packets after
vxlan_pick_source_port picks a source port during encapsulation.
Reviewed by: zlei, kp, adrian
Differential Revision: https://reviews.freebsd.org/D53022
(cherry picked from commit 1cc316727ebae157b3d035d9fb1ad38310a80698)