util: MAX_FROM_BITS() should be unsigned

MAX_FROM_BITS() computes the maximum value representable in a number of
bits.  The expression for that is an unsigned value, but we explicitly cast
it to a signed int.  It looks like this is because one of the main users is
for FD_REF_MAX, which is used to bound fd values, typically stored as a
signed int.

The value MAX_FROM_BITS() is calculating is naturally non-negative, though,
so it makes more sense for it to be unsigned, and to move the case to the
definition of FD_REF_MAX.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

passt.h

@@ -87,7 +87,7 @@ union epoll_ref {
 	struct {
 		enum epoll_type type:8;
 #define FD_REF_BITS		24
-#define FD_REF_MAX		MAX_FROM_BITS(FD_REF_BITS)
+#define FD_REF_MAX		((int)MAX_FROM_BITS(FD_REF_BITS))
 		int32_t		fd:FD_REF_BITS;
 		union {
 			union tcp_epoll_ref tcp;