9p: strlen() doesn't count the terminator

This is an off by one bug because strlen() doesn't count the NULL
terminator.  We strcpy() addr into a fixed length array of size
UNIX_PATH_MAX later on.

The addr variable is the name of the device being mounted.

CC: stable@kernel.org
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>