block/rnbd: correct all kernel-doc complaints

block/rnbd: correct all kernel-doc complaints

Fix all kernel-doc warnings in rnbd-proto.h:
- use correct enum name in kdoc comment
- mark several struct members as "/* private: */" so that no kdoc is
  required for them
- don't use "/**" for a non-kernel-doc comment
- use the correct struct member name for "dev_name"
- use " *" for a blank kernel-doc line

Fixes these warnings:
Warning: drivers/block/rnbd/rnbd-proto.h:41 expecting prototype for
 enum rnbd_msg_types. Prototype was for enum rnbd_msg_type instead
Warning: drivers/block/rnbd/rnbd-proto.h:50 struct member '__padding'
 not described in 'rnbd_msg_hdr'
Warning: drivers/block/rnbd/rnbd-proto.h:53 This comment starts with
 '/**', but isn't a kernel-doc comment.
 * We allow to map RO many times and RW only once. We allow to map yet another
Warning: drivers/block/rnbd/rnbd-proto.h:81 struct member 'reserved'
 not described in 'rnbd_msg_sess_info'
Warning: drivers/block/rnbd/rnbd-proto.h:92 struct member 'reserved'
 not described in 'rnbd_msg_sess_info_rsp'
Warning: drivers/block/rnbd/rnbd-proto.h:107 struct member 'resv1'
 not described in 'rnbd_msg_open'
Warning: drivers/block/rnbd/rnbd-proto.h:107 struct member 'dev_name'
 not described in 'rnbd_msg_open'
Warning: drivers/block/rnbd/rnbd-proto.h:107 struct member 'reserved'
 not described in 'rnbd_msg_open'
Warning: drivers/block/rnbd/rnbd-proto.h:158 struct member 'reserved'
 not described in 'rnbd_msg_open_rsp'
Warning: drivers/block/rnbd/rnbd-proto.h:189 bad line:

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>