pc-bios/s390-ccw: fix off-by-one error

This error takes effect when the magic value "zIPL" is located at the
end of a block. For example if s2_cur_blk = 0x7fe18000 and the magic
value "zIPL" is located at 0x7fe18ffc - 0x7fe18fff.

Fixes: ba831b25 ("s390-ccw: read stage2 boot loader data to find menu")
Reviewed-by: Collin Walling <walling@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Message-Id: <20200924085926.21709-2-mhartmay@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
[thuth: Use "<= ... - 4" instead of "< ... - 3"]
Signed-off-by: Thomas Huth <thuth@redhat.com>

pc-bios/s390-ccw/bootmap.c

@@ -163,7 +163,7 @@ static bool find_zipl_boot_menu_banner(int *offset)
     int i;
 
     /* Menu banner starts with "zIPL" */
-    for (i = 0; i < virtio_get_block_size() - 4; i++) {
+    for (i = 0; i <= virtio_get_block_size() - 4; i++) {
         if (magic_match(s2_cur_blk + i, ZIPL_MAGIC_EBCDIC)) {
             *offset = i;
             return true;