LU-3157 llite: A not locked mutex can be unlocked.

In case of memory pressure a not locked mutex can be unlocked
in function ll_file_open(). This is not allowed and subsequent
behavior is not defined.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I21368247cac23038f9b995e9de328f9286909731
Reviewed-on: http://review.whamcloud.com/6028
Reviewed-by: John Hammond <johnlockwoodhammond@gmail.com>
Reviewed-by: Nikitas Angelinas <nikitas_angelinas@xyratex.com>
Tested-by: Hudson
Reviewed-by: Sebastien Buisson <sebastien.buisson@bull.net>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>