DESC
uml: use always a separate io thread for UBD
EDESC
From: blaisorblade_spam@yahoo.it

Currently, ubd=sync is different from replacing ubd#= with ubd#s=. This is
against Principle of Least Surprise, so remove this difference.

Also the current ubd=sync behaviour is completely useless: it is to make sure
that when the kernel has synched its I/O to the virtual disk, the host does
not invalidate this with his caching; this causes ReiserFS corruption.

But since actually we call end_request() only after the io_thread has done its
work, we never lie to the block layer. Using O_SYNC as we do when replacing
ubd#= with ubd#s= is enough.

Also merged uml-ubd-thread-start-fail-well:

Correct the failure path in start_io_thread(), to return the correct
error code.