SUNRPC: Fix a server shutdown leak

SUNRPC: Fix a server shutdown leak

BugLink: https://bugs.launchpad.net/bugs/2016875
[ Upstream commit 9ca6705d9d609441d34f8b853e1e4a6369b3b171 ]

Fix a race where kthread_stop() may prevent the threadfn from ever getting
called.  If that happens the svc_rqst will not be cleaned up.

Fixes: ed6473ddc704 ("NFSv4: Fix callback server shutdown")
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>