vsprintf: deal with format specifiers with a lookup table

vsprintf: deal with format specifiers with a lookup table

We did the flags as an array earlier, they had simpler rules.  The final
format specifiers are a bit more complex since they have more fields to
deal with, and we want to handle the length modifiers at the same time.
But like the flags, we're better off just making it a data-driven table
rather than some case statement.

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