New libfs.c helper - simple_fill_super().  Abstracted from
nfsd/nfsctl.c, couple of filesystems converted to it (nfsctl, binfmt_misc).

	Function takes an array of triples (name, file_operations, mode),
superblock and value for its ->s_magic.  It acts as fill_super() - populates
superblock or fails.  We get a ramfs-style flat tree - root directory and
a bunch of files in it.

	That animal allows to put together a simple filesystem without
touching any directory-related stuff - now it's as easy as implementing
file_operations for files you want to have and telling what to call them.