LU-3953 config: move ext4 src detection to earlier phase

LU-3953 config: move ext4 src detection to earlier phase

Moving ext4 src detection to earlier in config phase to
make sure enable_ldiskfs is marked no to avoid build
failure from client only.

Correct syntax error in LB_ZFS_USER.

Signed-off-by: Minh Diep <minh.diep@intel.com>
Change-Id: I8b707dd66bcceb2b5b56a26ea7354c455c03f3d8
Reviewed-on: http://review.whamcloud.com/7692
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>