[S390] ptrace: fix return value of do_syscall_trace_enter()

strace may change the system call number, so regs->gprs[2] must not
be read before tracehook_report_syscall_entry(). This fixes a bug
where "strace -f" will hang after a vfork().

Cc: <stable@kernel.org>
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>