x86 signals: lift set_fs

This lifts the set_fs(USER_DS) call for signal handler setup out of the
three places copying the same code into the one place that calls them
all.  There is no change in what it does.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>