net: l2tp: fix clang -Wformat warning

net: l2tp: fix clang -Wformat warning

When building with clang we encounter this warning:
| net/l2tp/l2tp_ppp.c:1557:6: error: format specifies type 'unsigned
| short' but the argument has type 'u32' (aka 'unsigned int')
| [-Werror,-Wformat] session->nr, session->ns,

Both session->nr and session->ns are of type u32. The format specifier
previously used is `%hu` which would truncate our unsigned integer from
32 to 16 bits. This doesn't seem like intended behavior, if it is then
perhaps we need to consider suppressing the warning with pragma clauses.

This patch should get us closer to the goal of enabling the -Wformat
flag for Clang builds.

Link: https://github.com/ClangBuiltLinux/linux/issues/378
Signed-off-by: Justin Stitt <justinstitt@google.com>
Acked-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/20220706230833.535238-1-justinstitt@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>