ocfs2: try to reuse extent block in dealloc without meta_alloc

ocfs2: try to reuse extent block in dealloc without meta_alloc

A crash issue was reported by John Lightsey with a call trace as follows:

  ocfs2_split_extent+0x1ad3/0x1b40 [ocfs2]
  ocfs2_change_extent_flag+0x33a/0x470 [ocfs2]
  ocfs2_mark_extent_written+0x172/0x220 [ocfs2]
  ocfs2_dio_end_io+0x62d/0x910 [ocfs2]
  dio_complete+0x19a/0x1a0
  do_blockdev_direct_IO+0x19dd/0x1eb0
  __blockdev_direct_IO+0x43/0x50
  ocfs2_direct_IO+0x8f/0xa0 [ocfs2]
  generic_file_direct_write+0xb2/0x170
  __generic_file_write_iter+0xc3/0x1b0
  ocfs2_file_write_iter+0x4bb/0xca0 [ocfs2]
  __vfs_write+0xae/0xf0
  vfs_write+0xb8/0x1b0
  SyS_write+0x4f/0xb0
  system_call_fastpath+0x16/0x75

The BUG code told that extent tree wants to grow but no metadata was
reserved ahead of time.  From my investigation into this issue, the root
cause it that although enough metadata is not reserved, there should be
enough for following use.  Rightmost extent is merged into its left one
due to a certain times of marking extent written.  Because during
marking extent written, we got many physically continuous extents.  At
last, an empty extent showed up and the rightmost path is removed from
extent tree.

Add a new mechanism to reuse extent block cached in dealloc which were
just unlinked from extent tree to solve this crash issue.

Criteria is that during marking extents *written*, if extent rotation
and merging results in unlinking extent with growing extent tree later
without any metadata reserved ahead of time, try to reuse those extents
in dealloc in which deleted extents are cached.

Also, this patch addresses the issue John reported that ::dw_zero_count
is not calculated properly.

After applying this patch, the issue John reported was gone.  Thanks for
the reproducer provided by John.  And this patch has passed
ocfs2-test(29 cases) suite running by New H3C Group.

[ge.changwei@h3c.com: fix static checker warnning]
Link: http://lkml.kernel.org/r/63ADC13FD55D6546B7DECE290D39E373F29196AE@H3CMLB12-EX.srv.huawei-3com.com
[akpm@linux-foundation.org: brelse(NULL) is legal]
Link: http://lkml.kernel.org/r/1515479070-32653-2-git-send-email-ge.changwei@h3c.com
Signed-off-by: Changwei Ge <ge.changwei@h3c.com>
Reported-by: John Lightsey <john@nixnuts.net>
Tested-by: John Lightsey <john@nixnuts.net>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Joseph Qi <jiangqi903@gmail.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Mark Fasheh <mfasheh@versity.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>