NFS: Open-code the nfs_kset kset_create_and_add()

In preparation to make objects below /sys/fs/nfs namespace aware, we need
to define our own kobj_type for the nfs kset so that we can add the
.child_ns_type member in a following patch.  No functional change here, only
the unrolling of kset_create_and_add().

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>