um: Rewrite the sigio workaround based on epoll and tgkill

The existing sigio workaround implementation removes FDs from the
poll when events are triggered, requiring users to re-add them via
add_sigio_fd() after processing. This introduces a potential race
condition between FD removal in write_sigio_thread() and next_poll
update in __add_sigio_fd(), and is inefficient due to frequent FD
removal and re-addition. Rewrite the implementation based on epoll
and tgkill for improved efficiency and reliability.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250315161910.4082396-2-tiwei.btw@antgroup.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>