LU-723 ldiskfs: Drop support for ext3 based ldiskfs

LU-723 ldiskfs: Drop support for ext3 based ldiskfs

Building ldiskfs from ext3 is no longer supported, thus this change
attempts to clean things up and only support building ldiskfs from
ext4 sources. The main changes made, involved replacing the @BACKFS@
autoconf variable with 'ext4', and removing the HAVE_EXT4_LDISKFS
macro. It is now safe to assume that ldiskfs is build based entirely
on ext4.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: Ia0eff6e4c1b755806ccfbf554e4a36201f9f1a64
Reviewed-on: http://review.whamcloud.com/1643
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>