Share the "blocked-on file descriptor" number between read() and write().

A process can't be reading and writing at the same time, so it's fine for
them to share the variable for the blocked FD.

Kernel/Process.cpp

@@ -1055,7 +1055,7 @@ ssize_t Process::sys$read(int fd, void* outbuf, size_t nread)
         return -EBADF;
     if (descriptor->is_blocking()) {
         if (!descriptor->has_data_available_for_reading()) {
-            m_fdBlockedOnRead = fd;
+            m_blocked_fd = fd;
             block(BlockedRead);
             sched_yield();
             if (m_was_interrupted_while_blocked)