This patch fixes in st.c the bug in the signed/unsigned int comparison

reported by Doug Gilbert and fixed by him in sg.c (see [PATCH] sg direct
io/mmap oops). Doug fixed the comparison in sg.c. This fix for st.c does not
touch the comparison but makes both arguments signed to remove the
problem. The new code is adapted from linux/fs/bio.c.

Signed-off-by: Kai Makisara <kai.makisara@kolumbus.fi>

Rejections fixed up and

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>