ocfs2: test target page before change it

To support direct io in ocfs2_write_begin_nolock & ocfs2_write_end_nolock.

Direct io data will not appear in buffer.  The w_target_page member will
not be filled by direct io.  So avoid to use it when it's NULL.  Unlinke
buffer io and mmap, direct io will call write_begin with more than 1
page a time.  So the target_index is not sufficient to describe the
actual data.  change it to a range start at target_index, end in
end_index.

Signed-off-by: Ryan Ding <ryan.ding@oracle.com>
Reviewed-by: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Joseph Qi <joseph.qi@huawei.com>
Cc: Mark Fasheh <mfasheh@suse.de>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>