add rpc_status netlink support for NFSD

Introduce rpc_status netlink support for NFSD in order to dump pending RPC requests debugging information from userspace.
The new code can be tested with the user-space tool reported below:
https://github.com/LorenzoBianconi/nfsd-rpc-netlink-monitor/tree/main

Changes since v7:
- introduce nfsd_server.yaml for netlink messages

Changes since v6:
- report info to user-space through netlink and get rid of the related entry in the procfs

Changes since v5:
- add missing delimiters for nfs4 compound ops
- add a header line for rpc_status handler
- do not dump rq_prog
- do not dump rq_flags in hex

Changes since v4:
- rely on acquire/release APIs and get rid of atomic operation
- fix kdoc for nfsd_rpc_status_open
- get rid of ',' as field delimiter in nfsd_rpc_status hanlder
- move nfsd_rpc_status before nfsd_v4 enum entries
- fix compilantion error if nfsdv4 is not enabled

Changes since v3:
- introduce rq_status_counter in order to detect if the RPC request is pending and RPC info are stable
- rely on __svc_print_addr to dump IP info

Changes since v2:
- minor changes in nfsd_rpc_status_show output

Changes since v1:
- rework nfsd_rpc_status_show output

Changes since RFCv1:
- riduce time holding nfsd_mutex bumping svc_serv refcoung in nfsd_rpc_status_open()
- dump rqstp->rq_stime
- add missing kdoc for nfsd_rpc_status_open()

Lorenzo Bianconi (3):
Documentation: netlink: add a YAML spec for nfsd_server
NFSD: introduce netlink rpc_status stubs
NFSD: add rpc_status netlink support

 Documentation/netlink/specs/nfsd_server.yaml |  97 +++++++++
 fs/nfsd/Makefile                             |   3 +-
 fs/nfsd/nfs_netlink_gen.c                    |  32 +++
 fs/nfsd/nfs_netlink_gen.h                    |  22 ++
 fs/nfsd/nfsctl.c                             | 204 +++++++++++++++++++
 fs/nfsd/nfsd.h                               |  16 ++
 fs/nfsd/nfssvc.c                             |  15 ++
 fs/nfsd/state.h                              |   2 -
 include/linux/sunrpc/svc.h                   |   1 +
 include/uapi/linux/nfsd_server.h             |  49 +++++
 10 files changed, 438 insertions(+), 3 deletions(-)
 create mode 100644 Documentation/netlink/specs/nfsd_server.yaml
 create mode 100644 fs/nfsd/nfs_netlink_gen.c
 create mode 100644 fs/nfsd/nfs_netlink_gen.h
 create mode 100644 include/uapi/linux/nfsd_server.h

2.41.0