libceph, ceph: move ceph_osdc_copy_from() into cephfs code

This patch moves ceph_osdc_copy_from() function out of libceph code into
cephfs.  There are no other users for this function, and there is the need
(in another patch) to access internal ceph_osd_request struct members.

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>