There are at least 2 reports about memory bit flip sneaking into on-disk
data.

Currently we only have a relaxed check triggered at
btrfs_mark_buffer_dirty() time, as it's not mandatory and only for
CONFIG_BTRFS_FS_CHECK_INTEGRITY enabled build, it doesn't help user to
detect such problem.

This patch will address the hole by triggering comprehensive check on
tree blocks before writing it back to disk.

The design points are:
- Timing of the check: Tree block write hook
  This timing is chosen to reduce the overhead.
  The comprehensive check should be as expensive as csum.
  Doing full check at btrfs_mark_buffer_dirty() is too expensive for end
  user.

- Loose empty leaf check
  Originally for empty leaf, tree-checker will report error if it's not
  a tree root.
  The problem for such check at write time is:
  * False alert for tree root created in current transaction
    In that case, the commit root still needs to be written to disk.
    And since current root can differ from commit root, then it will
    cause false alert.
    This happens for log tree.

  * False alert for relocated tree block
    Relocated tree block can be written to disk due to memory pressure,
    in that case an empty csum tree root can be written to disk and
    cause false alert, since csum root node hasn't been updated.

  Although some more reliable empty leaf check is still kept as is.
  Namely essential trees (e.g. extent, chunk) should never be empty.

The example error output will be something like:
  BTRFS critical (device dm-3): corrupt leaf: root=2 block=1350630375424 slot=68, bad key order, prev (10510212874240 169 0) current (1714119868416 169 0)
  BTRFS error (device dm-3): block=1350630375424 write time tree block corruption detected
  BTRFS: error (device dm-3) in btrfs_commit_transaction:2220: errno=-5 IO failure (Error while writing out transaction)
  BTRFS info (device dm-3): forced readonly
  BTRFS warning (device dm-3): Skipping commit of aborted transaction.
  BTRFS: error (device dm-3) in cleanup_transaction:1839: errno=-5 IO failure
  BTRFS info (device dm-3): delayed_refs has NO entry

Reported-by: Leonard Lausen <leonard@lausen.nl>
Signed-off-by: Qu Wenruo <wqu@suse.com>