IPVS: fix potential stack overflow with overly long protocol names

When protocols use very long names, the sprintf calls might overflow
the on-stack buffer. No protocol in the kernel does this however.

Print the protocol name in the pr_debug statement directly to avoid
this.

Based on patch by Zhitong Wang <zhitong.wangzt@alibaba-inc.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Patrick McHardy <kaber@trash.net>