fs/ocfs2/file.c: fix wrong comment

Unwritten extent only exists for file systems which support holes.  But
the comment said was opposite meaning and also the comment is not very
clear, so rephase it.

Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>