zpool_disable_datasets: on Linux, detach mounts when forcing export

On Linux, MNT_FORCE makes the kernel inform that fileystem that its
about to call its unmount method so it can begin to eject active IO,
making it more likely that the unmount will succeed. This however does
not arrange for the unmount method to always succeed; new IO between the
two filesystem calls can dirty the filesystem. This is very difficult to
lock out properly within ZFS, as not all operations that cause the
kernel to dirty the filesystem can easily locked out (eg zfs_lookup).

So, we add MNT_DETACH as well. This causes the kernel to first remove
the mount from the user namespace, giving the appearance that it has
been unmounted (ie no longer appears in /proc/mounts), so that userspace
can't reference the filesystem anymore. The unmount then proceeds in the
background.

Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
(cherry picked from commit d2e1634fc935288aa851b5915feaa670c791265c)

lib/libzfs/libzfs_mount.c

@@ -1538,6 +1538,10 @@ zpool_disable_datasets(zpool_handle_t *zhp, boolean_t force,
 	int ret = -1;
 	int flags = ((hardforce || force) ? MS_FORCE : 0);
 
+#ifdef __linux__
+	if (hardforce) flags |= MS_DETACH;
+#endif
+
 	hdl->libzfs_force_export = flags & MS_FORCE;
 	namelen = strlen(zhp->zpool_name);