virfile: safezero: fix buffer allocation max size

My previous commit 7dc1d4ab was supposed to change safezero to allocate
1 megabyte at maximum, but had the logic reversed and will allocate 1
megabyte at minimum (and a lot more at maximum.)

Signed-off-by: Oskari Saarenmaa <os@ohmu.fi>

src/util/virfile.c

@@ -1075,7 +1075,7 @@ safezero(int fd, off_t offset, off_t len)
 
     /* Split up the write in small chunks so as not to allocate lots of RAM */
     remain = len;
-    bytes = MAX(1024 * 1024, len);
+    bytes = MIN(1024 * 1024, len);
 
     r = VIR_ALLOC_N(buf, bytes);
     if (r < 0) {