Cache tcp->u_nargs in a local variable for for() loops

Cache tcp->u_nargs in a local variable for for() loops

Loops of the form "for (i = 0; i < tcp->u_nargs; i++) ..."
need to fetch tcp->u_nargs from memory on every iteration
if "..." part has a function call (gcc doesn't know that
tcp->u_nargs won't change). This can be sped up
by putting tcp->u_nargs in a local variable, which might
go into a CPU register.

* syscall.c (decode_subcall): Cache tcp->u_nargs in a local variable
as for() loop limit value.
(syscall_enter): Likewise.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>