LU-11922 ldiskfs: make dirdata work with metadata_csum

LU-11922 ldiskfs: make dirdata work with metadata_csum

Handle ext4_dir_entry_tail correctly, which is a bogus dir entry
contains the checksum at the end of dir leaf block.

Fix how we get to the limit on the dx_root, we can't assume the
rec_len as 12 as . and .. in front of the dx_root have dirdata.

Also includes another fix for large_dir, where we should update
checksum for dx_node calling ext4_handle_dirty_dx_node(). The change
also makes large_dir patch more consistent with the upstream version.

With this we can enable metadata_csum on the targets.

Test-Parameters: fstype=ldiskfs envdefinitions=LDISKFS_MKFS_OPTS="-O metadata_csum"
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I04df8c30d9d423111e2b4031a7e4b9058101016f
Reviewed-on: https://review.whamcloud.com/34219
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>