ksmbd: add low bound validation to FSCTL_QUERY_ALLOCATED_RANGES

Smatch static checker warning:
 fs/ksmbd/vfs.c:1040 ksmbd_vfs_fqar_lseek() warn: no lower bound on 'length'
 fs/ksmbd/vfs.c:1041 ksmbd_vfs_fqar_lseek() warn: no lower bound on 'start'

Fix unexpected result that could caused from negative start and length.

Fixes: f4415848 ("cifsd: add file operations")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/ksmbd/smb2pdu.c

@@ -7448,13 +7448,16 @@ static int fsctl_query_allocated_ranges(struct ksmbd_work *work, u64 id,
 	if (in_count == 0)
 		return -EINVAL;
 
+	start = le64_to_cpu(qar_req->file_offset);
+	length = le64_to_cpu(qar_req->length);
+
+	if (start < 0 || length < 0)
+		return -EINVAL;
+
 	fp = ksmbd_lookup_fd_fast(work, id);
 	if (!fp)
 		return -ENOENT;
 
-	start = le64_to_cpu(qar_req->file_offset);
-	length = le64_to_cpu(qar_req->length);
-
 	ret = ksmbd_vfs_fqar_lseek(fp, start, length,
 				   qar_rsp, in_count, out_count);
 	if (ret && ret != -E2BIG)