ceph: add checking of wait_for_completion_killable() return value

The Coverity Scan service has detected the calling of
wait_for_completion_killable() without checking the return
value in ceph_lock_wait_for_completion() [1]. The CID 1636232
defect contains explanation: "If the function returns an error
value, the error value may be mistaken for a normal value.
In ceph_lock_wait_for_completion(): Value returned from
a function is not checked for errors before being used. (CWE-252)".

The patch adds the checking of wait_for_completion_killable()
return value and return the error code from
ceph_lock_wait_for_completion().

[1] https://scan5.scan.coverity.com/#/project-view/64304/10063?selectedIssue=1636232

Signed-off-by: Viacheslav Dubeyko <[e-mail]>
Reviewed-by: Alex Markuze <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

fs/ceph/locks.c

···
 	if (err && err != -ERESTARTSYS)
 		return err;
 
-	wait_for_completion_killable(&req->r_safe_completion);
+	err = wait_for_completion_killable(&req->r_safe_completion);
+	if (err)
+		return err;
+
 	return 0;
 }