jffs2reset: use jffs2_mark if rootfs_data isn't mounted

jffs2reset: use jffs2_mark if rootfs_data isn't mounted

Erasing all rootfs_data blocks may cause some problems with partition
identification. It won't contain MAGIC, but will be successfully mounted
with delayed blocks marking. This may be really confusing when user
reboots before JFFS2 finishes its blocks management. During the next
boot rootfs_data will be a valid partition (possibly with data) but
libblkid won't detect it.
Also adjust message in jffs2_mark to make more sense when used together
with jffs2_reset.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>