ceph: fix iov_iter issues in ceph_direct_read_write()

dio_get_pagev_size() and dio_get_pages_alloc() introduced in commit
b5b98989 ("ceph: combine as many iovec as possile into one OSD
request") assume that the passed iov_iter is ITER_IOVEC.  This isn't
the case with splice where it ends up poking into the guts of ITER_BVEC
or ITER_PIPE iterators, causing lockups and crashes easily reproduced
with generic/095.

Rather than trying to figure out gap alignment and stuff pages into
a page vector, add a helper for going from iov_iter to a bio_vec array
and make use of the new CEPH_OSD_DATA_TYPE_BVECS code.

Fixes: b5b98989 ("ceph: combine as many iovec as possile into one OSD request")
Link: http://tracker.ceph.com/issues/18130


Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Tested-by: Luis Henriques <lhenriques@suse.com>