jffs2: remove unused variables

Remove unused 'jffs2_sb_info *c' variable from 'jffs2_lookup()'
and 'jffs2_readdir()'.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>