ext4: fix build failure if DX_DEBUG is enabled

ext4_next_entry() is used by the debugging function dx_show_leaf(), so
it must be defined before that function.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>