vsprintf.c: Reduce sizeof struct printf_spec from 24 to 8 bytes

Reducing the size of struct printf_spec is a good thing because multiple
instances are commonly passed on stack.

It's possible for type to be u8 and field_width to be s8, but this is
likely small enough for now.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>