Currently the only non-blockdevice filesystems that can be used as the
initial root filesystem are NFS and CIFS, which use the magic
"root=/dev/nfs" and "root=/dev/cifs" syntax that requires the root
device file system details to come from filesystem specific kernel
command line options.

Add a little bit of new code that allows to just pass arbitrary
string mount options to any non-blockdevice filesystems so that it can
be mounted as the root file system.

For example a virtiofs root file system can be mounted using the
following syntax:

"root=myfs rootfstype=virtiofs rw"

Based on an earlier patch from Vivek Goyal <vgoyal@redhat.com>.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>