jffs2: Use pr_fmt and remove jffs: from formats

Use pr_fmt to prefix KBUILD_MODNAME to appropriate logging messages.

Remove now unnecessary internal prefixes from formats.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>