dounmount: do not call mountcheckdirs() for mounts with MNT_IGNORE

This is a bit hackish, but the flag is currently set only for ZFS
snapshots mounted under .zfs. mountcheckdirs() can change cdir/rdir
references to a covered vnode. But for the said snapshots the covered
vnode is really ephemeral and it must never be accessed (except
for a few specific cases).

To do: consider removing mountcheckdirs() entirely

MFC after: 5 days