powerpc/smp: poll cpu_callin_map more aggressively in __cpu_up()

At boot time, it is not necessary to delay between polls of
cpu_callin_map when waiting for a kicked CPU to come up. Remove the
delay intervals, but preserve the overall deadline (five seconds).

At run time, the first poll result is usually negative and we incur a
sleeping wait. If we spin on the callin word for a short time first,
we can reduce __cpu_up() from dozens of milliseconds to under 1ms in
the common case on a P9 LPAR:

$ ppc64_cpu --smt=off
$ bpftrace -e 'kprobe:__cpu_up {
                 @start[tid] = nsecs;
               }
               kretprobe:__cpu_up /@start[tid]/ {
                 @us = hist((nsecs - @start[tid]) / 1000);
                 delete(@start[tid]);
               }' -c 'ppc64_cpu --smt=on'

Before:

@us:
[16K, 32K)        85 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[32K, 64K)        13 |@@@@@@@                                             |

After:

@us:
[128, 256)        95 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[256, 512)         3 |@                                                   |

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220926220250.157022-1-nathanl@linux.ibm.com