NFSD: fix use-after-free in nfsd4_ssc_setup_dul()

NFSD: fix use-after-free in nfsd4_ssc_setup_dul()

[ Upstream commit 6ea49a482d3076ea54239d7f2172b14a20599794 ]

If signal_pending() returns true, schedule_timeout() will not be executed,
causing the waiting task to remain in the wait queue.
Fixed by adding a call to finish_wait(), which ensures that the waiting
task will always be removed from the wait queue.

Fixes: 006c81776132 ("NFSD: delay unmount source's export after inter-server copy completed.")
Signed-off-by: Xingyuan Mo <hdthky0@gmail.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>