vsnprintf: inline skip_atoi() again

vsnprintf: inline skip_atoi() again

At some point skip_atoi() had been marked 'noinline_for_stack', but it
turns out that this is now a pessimization, and not inlining it actually
results in a stack frame in format decoding due to the call and thus
hurts stack usage rather than helping.

With the simplistic atoi function inlined, the format decoding now needs
no frame at all.

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