um: series of __get_user() is costly in sigframe handling

It's not x86, where __get_user() is a single dereference; here it's
a single ptrace(2) call in host, which obviously costs a lot more.
IOW, it's cheaper to do copy_{to,from}_user() once than bother with
fields one by one...

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