lib: vsprintf: useless strlen() removed

The strict_strtoul() and strict_strtoull() functions used strlen() to
check argument's length in a situation where it wasn't strictly necessary

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Cc: "Yi Yang" <yi.y.yang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>