kernel/fd: Create the SyscallRestartWrapper before calling common_vector_io.

Otherwise, it seems compiler will optimize the assign operation into
a call of the first constructor, meaning the thread's syscall flag
won't be set properly.

Fixes #18951.

src/system/kernel/fs/fd.cpp

@@ -855,8 +855,8 @@ common_user_vector_io(int fd, off_t pos, const iovec* userVecs, size_t count,
 	if (error != B_OK)
 		return error;
 
-	SyscallRestartWrapper<ssize_t> result = common_vector_io(fd, pos,
-		vecs, count, write, false);
+	SyscallRestartWrapper<ssize_t> result;
+	result = common_vector_io(fd, pos, vecs, count, write, false);
 
 	return result;
 }