netfs, mm: Move PG_fscache helper funcs to linux/netfs.h

Move the PG_fscache related helper funcs (such as SetPageFsCache()) to
linux/netfs.h rather than linux/fscache.h as the intention is to move to a
model where they're used by the network filesystem and the helper library,
but not by fscache/cachefiles itself.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Dave Wysochanski <dwysocha@redhat.com>
Tested-By: Marc Dionne <marc.dionne@auristor.com>
cc: Matthew Wilcox <willy@infradead.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/161340392347.1303470.18065131603507621762.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/161539534516.286939.6265142985563005000.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653792959.2770958.5386546945273988117.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789073997.6155.18442271115255650614.stgit@warthog.procyon.org.uk/ # v6