shared/032: add options for jffs2

shared/032: add options for jffs2

Recently I found that my disk space was eaten by results/shared/032.full
after one round of xfstests running. The file was filled with binary
bytes by the command:
`mkfs -t jffs2 /dev/mapper/test-2 >> results/shared/032.full 2>&1`

Manual of mkfs.jffs2:
========================================================================
The program mkfs.jffs2 creates a JFFS2 (Second Journalling Flash
File System) file system image and writes the resulting image to the
file specified by the -o option or by default to the standard output,
unless the standard output is a terminal device in which case mkfs.jffs2
will abort. The file system image is created using the files and
directories contained in the directory specified by the option -r or
the present directory, if the -r option is not specified.
========================================================================

So the command equals to
`mkfs -t jffs2 -r xfstests-dev /dev/mapper/test-2 \
    -o xfstests-dev/results/shared/032.full \
    >>xfstests-dev/results/shared/032.full 2>&1`

The output image 032.full is under the directory xfstests-dev.
mkfs.jffs2 keeps reading from 032.full in page size and append bytes
to 032.full until the disk where xfstests-devel located is full.

Fix it by setting @preargs to '-r /proc/fs -o' for jffs2, now the output
is $SCRATCH_DEV. Since mkfs.jffs2 doesn't check overwrite case, just
skip overwrite attempt.

Signed-off-by: Su Yue <l@damenly.su>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>