spfs: do not umount original mount
Spfs remembers original spfs mount id (update_spfs_info) and later
checks it (is_mnt_fd) on fd/map/cwd/root/exe replacement, so fd openned
on bindmount of spfs is not swapped back to nfs. We can workaround it by
leaving original spfs mount in restored mntns.
note: Right fix here should be to collect in spfs all ids of all spfs
mounts belonging to one superblock and replace all fds from all those
mounts with fds on nfs from corresponding nfs mount. Now we only fix
degradation which appeared after switching criu mount-v2 to mainstream
mount-v2. Problem that spfs replaces fds only on one mount is out of
scope of this fix.
Fixes: ce34b982a ("spfs: do mount request from mount namespace of nfs mount")
Signed-off-by: Pavel Tikhomirov <firstname.lastname@example.org>