NFSD: Add NFSD_CMD_UNLOCK_FILESYSTEM netlink command

NFSD: Add NFSD_CMD_UNLOCK_FILESYSTEM netlink command

Add NFSD_CMD_UNLOCK_FILESYSTEM as a dedicated netlink command for
revoking NFS state under a filesystem path, providing a netlink
equivalent of /proc/fs/nfsd/unlock_fs.

The command requires a "path" string attribute containing the
filesystem path whose state should be released. The handler
resolves the path to its superblock, then cancels async copies,
releases NLM locks, and revokes NFSv4 state on that superblock.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>