ext4: allocate delalloc blocks before changing journal mode

delalloc blocks should be allocated before changing journal mode,
otherwise they can not be allocated and even more truncate on
delalloc blocks could triggre BUG by flushing delalloc buffers.

Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>