[Trusty SRU/Utopic] Avoid task hung messages caused by bcache writeback

[Stefan Bader]

This is cheating a little bit as only the second patch is strictly needed
to avoid the hung task. But I was able to cause an oops when playing around
with bcache and writing to writeback_running. This would be patch #3 which
is not in Linus tree but was accepted by the developers. And since patch
#1 looks to be adressing the exactly same problem somewhere else, I
guessed I could just throw it into the bundle. Patches #1 and #2 are in
Linus tree (as of 3.17-rc1). The were not marked stable and I asked about
that today. But at least the hung task gets fixed and bcache seems to be
operational in Trusty and Utopic with those (checked in a VM).

-Stefan