cifs: Don't support ITER_XARRAY

There's now no need to support ITER_XARRAY in cifs as netfslib hands down
ITER_FOLIOQ instead - and that's simpler to use with iterate_and_advance()
as it doesn't hold the RCU read lock over the step function.

This is part of the process of phasing out ITER_XARRAY.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Tom Talpey <tom@talpey.com>
cc: Enzo Matsumiya <ematsumiya@suse.de>
cc: linux-cifs@vger.kernel.org
Link: https://lore.kernel.org/r/20240814203850.2240469-26-dhowells@redhat.com/

 # v2
Signed-off-by: Christian Brauner <brauner@kernel.org>