libceph: ceph_decode_skip_* helpers

Some of these won't be as efficient as they could be (e.g.
ceph_decode_skip_set(... 32 ...) could advance by len * sizeof(u32)
once instead of advancing by sizeof(u32) len times), but that's fine
and not worth a bunch of extra macro code.

Replace skip_name_map() with ceph_decode_skip_map as an example.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>