Kernel: Return bytes written if sys$write() fails after writing some

If we wrote anything we should just inform userspace that we did,
and not worry about the error code. Userspace can call us again if
it wants, and we'll give them the error then.

Kernel/Process.cpp

@@ -1695,8 +1695,8 @@ ssize_t Process::do_write(FileDescription& description, const u8* data, int data
         dbg() << "   -> write returned " << rc;
 #endif
         if (rc < 0) {
-            // FIXME: Support returning partial nwritten with errno.
-            ASSERT(nwritten == 0);
+            if (nwritten)
+                return nwritten;
             return rc;
         }
         if (rc == 0)