ext4: use mkfs.ext4 -F instead of piping in yes

ext4: use mkfs.ext4 -F instead of piping in yes

Using "yes | mkfs.ext4 ..." results in the error message results in
the test failing, at least for some versions of e2fsprogs:

    +yes: standard output: Broken pipe
    +yes: write error

It better to use the -F option, which will eliminate the questions.

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