um: chan_user: catch EINTR when reading and writing

If the read/write function returns an error then we expect to see an
event/IRQ later on. However, this will only happen after an EAGAIN as we
are using edge based event triggering.

As such, EINTR needs to be caught should it happen.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20231018123643.1255813-2-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>