Spurious/undocumented EINTR from io_uring_enter

[Joseph Christopher Sible <jcsible@cert.org>]

When a process is blocking in io_uring_enter, and a signal stops it for
any reason, it returns -EINTR to userspace. Two comments about this:

1. https://github.com/axboe/liburing/blob/master/man/io_uring_enter.2
   doesn't mention EINTR as a possible error that it can return.
2. When there's no signal handler, and a signal stopped the syscall for
   some other reason (e.g., SIGSTOP, SIGTSTP, or any signal when the
   process is being traced), other syscalls (e.g., read) will be
   restarted transparently, but this one will return to userspace
   with -EINTR just as if there were a signal handler.

Point 1 seems like a no-brainer. I'm not sure if point 2 is possible
to fix, though, especially since some other syscalls (e.g., epoll_wait)
have the same problem as this one.

Joseph C. Sible