Implement poll to make select work correctly

[weiso131]

Using ftrace, I observed that during the select system call, the kernel invokes do_select → select_poll_one → vfs_poll → file->f_op->poll. However, the kxo device did not implement the poll file operation.

In this case, vfs_poll returns a default mask that causes select to assume the file is always ready for I/O, even if a subsequent read would block.

This behavior led to user-facing issues in xo-user. For example, attempting to pause (Ctrl+P) or
stop (Ctrl+Q) could be delayed or ignored due to
the blocking nature of kxo's read when no data is available.

I implemented poll support using a wait queue to resolve this problem.

References:

• github.com/torvalds/linux/blob/master/fs/select.c
• github.com/torvalds/linux/blob/master/include/linux/poll.h

[jserv]

Remove the following in git commit messages:

References:
- github.com/torvalds/linux/blob/master/fs/select.c
- github.com/torvalds/linux/blob/master/include/linux/poll.h

The above is less informative since developer can look up by themself.

[weiso131]

I've removed the References section from the commit message as suggested.

[jserv]

Thank @weiso131 for contributing!