libceph: handle OSD op ceph_pagelist_append() errors

osd_req_op_cls_init() and osd_req_op_xattr_init() currently propagate
ceph_pagelist_alloc() ENOMEM errors but ignore ceph_pagelist_append()
memory allocation failures. Add these checks and cleanup on error.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>