ext4: define MKFS_EXT4_PROG

And use it instead of "mkfs.ext4" / "mkfs -t ext4"

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

common/config

@@ -210,6 +210,7 @@ case "$HOSTOS" in
         ;;
     Linux)
         export MKFS_XFS_PROG="`set_prog_path mkfs.xfs`"
+        export MKFS_EXT4_PROG="`set_prog_path mkfs.ext4`"
         export MKFS_UDF_PROG="`set_prog_path mkudffs`"
         export MKFS_BTRFS_PROG="`set_btrfs_mkfs_prog_path_with_opts`"
         export BTRFS_UTIL_PROG="`set_prog_path btrfs`"