repart: fix free area calculations for unaligned partitions

To properly detect how much space we have to distribute we need to take
into account that both the partition offset and the partition size
aren't aligned.

src/partition/repart.c

@@ -505,18 +505,21 @@ static uint64_t free_area_available_for_new_partitions(const FreeArea *a) {
 
         avail = free_area_available(a);
         if (a->after) {
-                uint64_t need, space;
+                uint64_t need, space_end, new_end;
 
                 need = partition_min_size_with_padding(a->after);
 
                 assert(a->after->offset != UINT64_MAX);
                 assert(a->after->current_size != UINT64_MAX);
 
-                space = round_up_size(a->after->offset + a->after->current_size, 4096) - a->after->offset + avail;
-                if (need >= space)
-                        return 0;
+                /* Calculate where the free area ends, based on the offset of the partition preceding it */
+                space_end = round_up_size(a->after->offset + a->after->current_size, 4096) + avail;
 
-                return space - need;
+                /* Calculate where the partition would end when we give it as much as it needs */
+                new_end = round_up_size(a->after->offset + need, 4096);
+
+                /* Calculate saturated difference of the two: that's how much we have free for other partitions */
+                return LESS_BY(space_end, new_end);
         }
 
         return avail;