nfsd: avoid a NULL dereference in __cld_pipe_upcall()

nfsd: avoid a NULL dereference in __cld_pipe_upcall()

[ Upstream commit df60446cd1fb487becd1f36f4c0da9e0e523c0cf ]

If the rpc_pipefs is unmounted, then the rpc_pipe->dentry becomes NULL
and dereferencing the dentry->d_sb will trigger an oops.  The only
reason we're doing that is to determine the nfsd_net, which could
instead be passed in by the caller.  So do that instead.

Fixes: 11a60d159259 ("nfsd: add a "GetVersion" upcall for nfsdcld")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>