cifs: Add some helper functions

Add some helper functions to manipulate the folio marks by iterating
through a list of folios held in an xarray rather than using a page list.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org

Link: https://lore.kernel.org/r/164928616583.457102.15157033997163988344.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165211418840.3154751.3090684430628501879.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165348878940.2106726.204291614267188735.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165364825674.3334034.3356201708659748648.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/166126394799.708021.10637797063862600488.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/166697258147.61150.9940790486999562110.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166732030314.3186319.9209944805565413627.stgit@warthog.procyon.org.uk/ # rfc
Signed-off-by: Steve French <stfrench@microsoft.com>