NFSD: fix leaked reference count of nfsd4_ssc_umount_item

NFSD: fix leaked reference count of nfsd4_ssc_umount_item

BugLink: https://bugs.launchpad.net/bugs/2011425
[ Upstream commit 34e8f9ec4c9ac235f917747b23a200a5e0ec857b ]

The reference count of nfsd4_ssc_umount_item is not decremented
on error conditions. This prevents the laundromat from unmounting
the vfsmount of the source file.

This patch decrements the reference count of nfsd4_ssc_umount_item
on error.

Fixes: f4e44b393389 ("NFSD: delay unmount source's export after inter-server copy completed.")
Signed-off-by: Dai Ngo <dai.ngo@oracle.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: Andrea Righi <andrea.righi@canonical.com>