_hashsplit.c: switch rollsum_roll() to a macro instead of an inline function.

_hashsplit.c: switch rollsum_roll() to a macro instead of an inline function.

gcc 4.3's optimizer manages to fail at optimizing the inline, but works okay
with the macro.

Mysteriously, if find_ofs() is *not* static (and therefore presumably
*harder* to optimize), the optimizer works either way.  But removing the
static is just wrong, so use the macro instead.

The difference in speed is about 53 megs/sec vs 80 megs/sec on my machine
for this command:

	bup random 100M 2>/dev/null | bup split -N --bench

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>