[PATCH] 9p: null terminate error strings for debug print

We weren't properly NULL terminating protocol error strings for our debug
printk resulting in garbage being included in the output when debug was
enabled.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>