ceph: define ceph_encode_8_safe()

It's kind of a silly macro, but ceph_encode_8_safe() is the only one
missing from an otherwise pretty complete set.  It's not used, but
neither are a couple of the others in this set.

While in there, insert some whitespace to tidy up the alignment of
the line-terminating backslashes in some of the macro definitions.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>