lib/vsprintf.c: fix handling of %zd when using ssize_t

Documentation/printk-formats.txt says to use %zd for a ssize_t argument
and some drivers do.  Unfortunately this prints a positive number for
negative values eg:

  tpm_tis 70030000.tpm_tis: tpm_transmit: tpm_send: error 4294967234

Add a case to va_args a ssize_t type if the interpretation should be
signed.

Tested on PPC32.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>