ipv6: remove redundant check

err varibale will be set everytime,like -ENOBUFS and in if (err < 0),
 when code gets into this path. This check will just slowdown
the execution and that's all.

Signed-off-by: Xi Huang <xuiagnh@gmail.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Link: https://patch.msgid.link/20240820115442.49366-1-xuiagnh@gmail.com


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