s390: Initialize psw mask in perf_arch_fetch_caller_regs()

Also initialize regs->psw.mask in perf_arch_fetch_caller_regs().
This way user_mode(regs) will return false, like it should.

It looks like all current users initialize regs to zero, so that this
doesn't fix a bug currently. However it is better to not rely on callers
to do this.

Fixes: 914d52e4 ("s390: implement perf_arch_fetch_caller_regs")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>