um: Simplify IRQ handling code

Reduce dynamic allocations (and thereby cache misses) by simply
embedding the registration data for IRQs in the irq_entry, we
never supported these being really dynamic anyway as only one
was ever allowed ("Trying to reregister ...").

Lockless behaviour is preserved by removing the FD from the poll
set appropriately, but we use reg->events to indicate whether or
not this entry is used, rather than dynamically allocating them.

Also port the list of IRQ entries to list_head instead of the
current open-coded singly-linked list implementation, just for
sanity.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>