netfs: Make a netfs helper module

Make a netfs helper module to manage read request segmentation, caching
support and transparent huge page support on behalf of a network
filesystem.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-and-tested-by: Jeff Layton <jlayton@redhat.com>
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/160588496284.3465195.10102643717770106661.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161118135638.1232039.1622182202673126285.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161161031028.2537118.1213974428943508753.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/161340391427.1303470.14884950716721956560.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/161539531569.286939.18317119181653706665.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653790328.2770958.6710423217716151549.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789071202.6155.16519256513958534906.stgit@warthog.procyon.org.uk/ # v6