gdb/linux-nat: remove check based on current_inferior in linux_handle_extended_wait

gdb/linux-nat: remove check based on current_inferior in linux_handle_extended_wait

The check removed by this patch, using current_inferior, looks wrong.
When debugging multiple inferiors with the Linux native target and
linux_handle_extended_wait is called, there's no guarantee about which
is the current inferior.  The vfork-done event we receive could be for
any inferior.  If the vfork-done event is for a non-current inferior, we
end up wrongfully ignoring it.  As a result, the core never processes a
TARGET_WAITKIND_VFORK_DONE event, program_space::breakpoints_not_allowed
is never cleared, and breakpoints are never reinserted.  However,
because the Linux native target decided to ignore the event, it resumed
the thread - while breakpoints out.  And that's bad.

The proposed fix is to remove this check.  Always report vfork-done
events and let infrun's logic decide if it should be ignored.  We don't
save much cycles by filtering the event here.

Add a test that replicates the situation described above.  See comments
in the test for more details.

Change-Id: Ibe33c1716c3602e847be6c2093120696f2286fbf