Prevent possible deadlock in acct_gather_filesystem/lustre

Add missing unlock before return. Coverity 44888.

NEWS

@@ -37,6 +37,7 @@ documents those changes that are of interest to users and administrators.
  -- Fix proctrack/lua plugin to prevent possible deadlock.
  -- Prevent infinite loop in slurmstepd if execve fails.
  -- Prevent multiple responses to REQUEST_UPDATE_JOB_STEP message.
+ -- Prevent possible deadlock in acct_gather_filesystem/lustre on error.
 
 * Changes in Slurm 15.08.11
 ===========================

src/plugins/acct_gather_filesystem/lustre/acct_gather_filesystem_lustre.c

@@ -310,6 +310,7 @@ static int _update_node_filesystem(void)
 		if (dataset_id == SLURM_ERROR) {
 			error("FileSystem: Failed to create the dataset "
 			      "for Lustre");
+			slurm_mutex_unlock(&lustre_lock);
 			return SLURM_ERROR;
 		}