In commit d4d67150, we reopened an old hole for a 64-bit ptracer touching a
32-bit tracee in system call entry.  A %rax value set via ptrace at the
entry tracing stop gets used whole as a 32-bit syscall number, while we
only check the low 32 bits for validity.

Fix it by truncating %rax back to 32 bits after syscall_trace_enter,
in addition to testing the full 64 bits as has already been added.
Reported-by: NBen Hawkes <hawkes@sota.gen.nz>
Signed-off-by: NRoland McGrath <roland@redhat.com>
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>