invent_group_ids(): zero ->mnt_group_id always implies !IS_MNT_SHARED()

invent_group_ids(): zero ->mnt_group_id always implies !IS_MNT_SHARED()

All places where we call set_mnt_shared() are guaranteed to have
non-zero ->mnt_group_id - either by explicit test, or by having
done successful invent_group_ids() covering the same mount since
we'd grabbed namespace_sem.

The opposite combination (non-zero ->mnt_group_id and !IS_MNT_SHARED())
*is* possible - it means that we have allocated group id, but didn't
get around to set_mnt_shared() yet; such state is transient -
by the time we do namespace_unlock(), we must either do set_mnt_shared()
or unroll the group id allocations by cleanup_group_ids().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>