UML kernel does not exit cleanly from a shell script

[nathanchance]

See my upstream report for more information:

https://lore.kernel.org/r/Yjt31seiNv18HYrf@dev-arch.thelio-3990X/

This blocks #59 for now.

[nathanchance]

We should be able to revert our workaround soon:

https://lore.kernel.org/32824a71109fe3387d582abbf56601fb08bdc9ef.camel@sipsolutions.net/

[nathanchance]

The fix is now in mainline so I will revert our workaround: https://git.kernel.org/linus/57ae0b67b747031bc41fb44643aa5344ab58607e