masking a 64-bit value with a 32-bit mask that's zero-extended will turn off
many more bits than intended. make the mask the right length to keep all the high-order 1s.

Reference: /n/sources/patch/applied/aquarela-mask
Date: Thu Feb 28 18:55:35 CET 2013
Signed-off-by: charles.forsyth@gmail.com

--- /sys/src/cmd/aquarela/smbcommon.c	Thu Feb 28 18:54:45 2013
+++ /sys/src/cmd/aquarela/smbcommon.c	Thu Feb 28 18:54:41 2013
@@ -290,7 +290,7 @@
 vlong
 smbl2roundupvlong(vlong v, int l2)
 {
-	ulong mask;
+	uvlong mask;
 	mask = (1 << l2) - 1;
 	return (v + mask) & ~mask;
 }