initmpfs: move rootfs code from fs/ramfs/ to init/

initmpfs: move rootfs code from fs/ramfs/ to init/

When the rootfs code was a wrapper around ramfs, having them in the same
file made sense.  Now that it can wrap another filesystem type, move it in
with the init code instead.

This also allows a subsequent patch to access rootfstype= command line
arg.

Signed-off-by: Rob Landley <[email protected]>
Cc: Jeff Layton <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Stephen Warren <[email protected]>
Cc: Rusty Russell <[email protected]>
Cc: Jim Cromie <[email protected]>
Cc: Sam Ravnborg <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>