net: mctp: Fix bad kfree_skb in bind lookup test

The kunit test's skb_pkt is consumed by mctp_dst_input() so shouldn't be
freed separately.

Fixes: e6d8e7db ("net: mctp: Add bind lookup test")
Reported-by: Alexandre Ghiti <alex@ghiti.fr>
Closes: https://lore.kernel.org/all/734b02a3-1941-49df-a0da-ec14310d41e4@ghiti.fr/


Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://patch.msgid.link/20250812-fix-mctp-bind-test-v1-1-5e2128664eb3@codeconstruct.com.au


Signed-off-by: Jakub Kicinski <kuba@kernel.org>