jffs2: Fix mounting under new mount API

The mounting of jffs2 is broken due to the changes from the new mount API
because it specifies a "source" operation, but then doesn't actually
process it.  But because it specified it, it doesn't return -ENOPARAM and
the caller doesn't process it either and the source gets lost.

Fix this by simply removing the source parameter from jffs2 and letting the
VFS deal with it in the default manner.

To test it, enable CONFIG_MTD_MTDRAM and allow the default size and erase
block size parameters, then try and mount the /dev/mtdblock<N> file that
that creates as jffs2.  No need to initialise it.

Fixes: ec10a24f ("vfs: Convert jffs2 to use the new mount API")
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: David Woodhouse <dwmw2@infradead.org>
cc: Richard Weinberger <richard@nod.at>
cc: linux-mtd@lists.infradead.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>