net: off by one in inet6_pton()

If "scope_len" is sizeof(scope_id) then we would put the NUL terminator
one space beyond the end of the buffer.

Fixes: b1a951fe

 ("net/utils: generic inet_pton_with_scope helper")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>