mctp: test: zero out sockaddr

MCTP now requires that padding bytes are zero.

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Fixes: 1e4b50f0 ("mctp: handle the struct sockaddr_mctp padding fields")
Link: https://lore.kernel.org/r/20220110021806.2343023-1-matt@codeconstruct.com.au


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