netfs: Make CONFIG_NETFS_SUPPORT auto-selected rather than manual

Make the netfs helper library selected automatically by the things that use
it rather than being manually configured, even though it's required[1].

Fixes: 3a5829fe

 ("netfs: Make a netfs helper module")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-mm@kvack.org
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/CAMuHMdXJZ7iNQE964CdBOU=vRKVMFzo=YF_eiwsGgqzuvZ+TuA@mail.gmail.com [1]
Link: https://lore.kernel.org/r/162090298141.3166007.2971118149366779916.stgit@warthog.procyon.org.uk # v1