Fix get_zfs_sb race with concurrent umount

Certain ioctl operations will call get_zfs_sb, which will holds an active
count on sb without checking whether it's active or not. This will result
in use-after-free. We fix this by using atomic_inc_not_zero to make sure
we got an active sb.

P1                                          P2
---                                         ---
deactivate_locked_super(): s_active = 0
                                            zfs_sb_hold()
                                            ->get_zfs_sb(): s_active = 1
->zpl_kill_sb()
-->zpl_put_super()
--->zfs_umount()
---->zfs_sb_free(zsb)
                                            zfs_sb_rele(zsb)

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/zfs_ioctl.c

@@ -1379,9 +1379,9 @@ get_zfs_sb(const char *dsname, zfs_sb_t **zsbp)
 
 	mutex_enter(&os->os_user_ptr_lock);
 	*zsbp = dmu_objset_get_user(os);
-	if (*zsbp && (*zsbp)->z_sb) {
-		atomic_inc(&((*zsbp)->z_sb->s_active));
-	} else {
+	/* bump s_active only when non-zero to prevent umount race */
+	if (*zsbp == NULL || (*zsbp)->z_sb == NULL ||
+	    !atomic_inc_not_zero(&((*zsbp)->z_sb->s_active))) {
 		error = SET_ERROR(ESRCH);
 	}
 	mutex_exit(&os->os_user_ptr_lock);