vsnprintf: fix up kerneldoc for argument name changes

vsnprintf: fix up kerneldoc for argument name changes

Stephen Rothwell reports that I missed fixing up the documentation when
the argument names changed in commit 938df695e98d ("vsprintf: associate
the format state with the format pointer"), resulting in htmldoc
warnings like

  lib/vsprintf.c:2760: warning: Function parameter or struct member 'fmt_str' not described in 'vsnprintf'
  lib/vsprintf.c:2760: warning: Excess function parameter 'fmt' description in 'vsnprintf'
  ...

which I didn't notice because the doc build takes longer than the whole
"real" kernel build for me, so I never bother (and judging by the other
warnings, pretty much nobody else does either).

I guess the bigger issues won't be fixed until the doc build is much
faster (narrator: "That isn's in the cards") but at least linux-next
finds the new cases.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 938df695e98d ("vsprintf: associate the format state with the format pointer")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>