[21.02] netifd: backport "system-linux: fix deletion of ip tunnels (FS#4058)"

[Sven Roederer]

Hi,

recently found the time to update my router to OpenWrt-21.02. All seems fine 
beside an issue with IPIP-tunnels.
I was able to track this down to netifd. And I also found FS#4058 and the 
recent fix () pushed by Hans.

As the patch cleanly applies to current 21.02-netifd, it would be great to 
cherry-pick it to fix netifd.
relevant commit is: https://git.op enwrt.org/?p=project/netifd.git;a=commit 
;h=8f82742ca4f47f459284f3a07323d04da72ea 5f6


Best Sven