trace: introduce missing mutex_unlock()

Impact: fix tracing buffer mutex leak in case of allocation failure

This error was spotted by this semantic patch:

  http://www.emn.fr/x-info/coccinelle/mut.html



It looks correct as far as I can tell. Please review.

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>