if_vxlan(4): Add missing statistic for input packets

Event: Aberdeen hackathon 2022
Reviewed by: bryanv, kp
Approved by: kp (mentor)
Differential Revision: https://reviews.freebsd.org/D36841

(cherry picked from commit 1fc839f48921919dea386966e0ce4f20c26dcecf)
(cherry picked from commit 00399a54c561e625e6674ecf3b12e794aa712d30)