ocfs2: Fix contiguousness check in ocfs2_try_to_merge_extent_map()

The wrong member was compared in the continguousness check.

Acked-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>