UBI: fix eraseblock picking criteria

commit 7eb3aa65853e1b223bfc786b023b702018cb76c0 upstream.

The 'find_wl_entry()' function expects the maximum difference as the second
argument, not the maximum absolute value. So the "unknown" eraseblock picking
was incorrect, as Shmulik Ladkani spotted. This patch fixes the issue.

Reported-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>