[DO NOT MERGE] ceph: switch DIO code to use iov_iter_get_pages_alloc

[DO NOT MERGE] ceph: switch DIO code to use iov_iter_get_pages_alloc

xfstest generic/095 triggers soft lockups in kcephfs. It uses fio to
drive some I/O via vmsplice ane splice. Ceph then ends up trying to
access an ITER_BVEC type iov_iter as a ITER_IOVEC one. That causes it to
pick up a wrong offset and get stuck in an infinite loop while trying to
populate the page array. dio_get_pagev_size has a similar problem.

Now that iov_iter_get_pages_alloc doesn't stop after the first vector in
the array, we can just call it instead and dump the old code that tried
to do the same thing.

Signed-off-by: Jeff Layton <jlayton@redhat.com>