ext4: use EXT4FS_DEBUG instead of EXT4_DEBUG in fsync.c

We have EXT4FS_DEBUG for some old debug and CONFIG_EXT4_DEBUG
for the new mballoc debug, but there isn't any EXT4_DEBUG.

As CONFIG_EXT4_DEBUG seems to be only used in mballoc, use
EXT4FS_DEBUG in fsync.c.

[ It doesn't really matter; although I'm including this commit for
  consistency's sake.  The whole point of the #ifdef's is to disable
  the debugging code.  In general you're not going to want to enable
  all of the code protected by EXT4FS_DEBUG at the same time.  -- Ted ]

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>