libceph: specify position of extent operation

allow specifying position of extent operation in multi-operations
osd request. This is required for cephfs to convert inline data to
normal data (compare xattr, then write object).

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@redhat.com>

diff --cc fs/ceph/addr.c
Simple merge

diff --cc fs/ceph/file.c
Simple merge

diff --cc include/linux/ceph/osd_client.h
Simple merge

diff --cc net/ceph/osd_client.c
Simple merge