include/linux/kernel.h: add __must_check to strict_strto*()

The whole point to using the strict functions is to check the return
value.  If you don't, strict_strto*() will return you uninitialised
garbage.  Offenders have been observed in the wild.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>