Fix unconditional jump from list_lock_held()

Yes, it bit me, finally :-)

I suddenly got memory leaks and valgrind reporting "Conditional jump or move
depends on uninitialised value(s)" at the list_lock_held() loop from
add_event_source() - and _only_ there which spells trouble. This faulty jump
caused a premature exit (in all scenarios I tested, it may be different on
other machines, AFAICS) from the loop resulting in memory leaks because the
new event source wasn't added to the list and was thus lost. The fd was,
however, already registered with epoll() and everything else did work,
except for the automatic cleanup which needs the 'events' list in its
integrity.

The disassembly show the reason: __make_anon_int() does

   0x00000000004068e6 <+222>:   lea    0x8(%rsp),%rax
   0x00000000004068eb <+227>:   add    $0xf,%rax
   0x00000000004068ef <+231>:   shr    $0x4,%rax
   0x00000000004068f3 <+235>:   shl    $0x4,%rax
   0x00000000004068f7 <+239>:   movl   $0x1,(%rax)

to initialise the anonymous int while __get_anon_int() compiles to

   0x000000000040691a <+274>:   lea    0x8(%rsp),%rax

Obviously, both values can be different depending on the stack layout and
that was the case in add_event_source().

In a test run, I debugged __make_anon_int() == 0x7fffffffe3b0 and
__get_anon_int() == 0x7fffffffe3a8 so using 'long' instead of 'int' wouldn't
fix it either and I just won't using assembly or gcc's register-bound
variables because that's just equally 'assembly'.

I though about this problem quite some time and came to the obvious
solution: use __make_anon_int() only to allocate space and __get_anon_int()
to refer to the variable there. This way, the variable address is always
calculated the same way.

BTW: I wrote a patch to the Linux kernel yesterday which would even get me
     rid of the 'events' list because actually everything is still in the
     epoll interface but there is no way to unconditionally (without epoll
     event) get my private data back - which is just what my patch adds.
     I'll test it further and try to get into. Just hope it's not considered
     overkill because it _can_ be done in userspace...