s390: do not bypass BPENTER for interrupt system calls

s390: do not bypass BPENTER for interrupt system calls

BugLink: http://bugs.launchpad.net/bugs/1754580
The system call path can be interrupted before the switch back to the
standard branch prediction with BPENTER has been done. The critical
section cleanup code skips forward to .Lsysc_do_svc and bypasses the
BPENTER. In this case the kernel and all subsequent code will run with
the limited branch prediction.

Fixes: eacf67eb9b32 ("s390: run user space and KVM guests with modified branch prediction")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
(cherry picked from commit d5feec04fe578c8dbd9e2e1439afc2f0af761ed4)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>