Revert "um: Fix get_signal() usage"

Commit db2f24dc
was plain wrong. I did not realize the we are
allowed to loop here.
In fact we have to loop and must not return to userspace
before all SIGSEGVs have been delivered.
Other archs do this directly in their entry code, UML
does it here.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>