Clean-up: ust-consumer: simplify metadata cache unlock on error path

The metadata cache lock can be released directly after the call to
consumer_metadata_cache_write() since nothing depends on the cache
being locked after.

This simplifies the unlocking of the metadata cache by removing an
extra "unlock" that is specific to an error path.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ib0151d9640309005dfc83e95efb538e3c4f0df4a