fix default __strnlen_user macro

The existing __strnlen_user macro simply resolved to strnlen. However, the
count returned by strnlen_user should include the NULL byte. This patch
fixes the __strnlen_user macro to include the NULL byte in the count.

Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>