[S390] avoid warning in show_cpuinfo

[S390] avoid warning in show_cpuinfo

The .start function and indirectly the .next function of the show_cpuinfo
sequential operation uses NR_CPUS as limit instead of nr_cpu_ids.
This can cause warnings like this:

WARNING: at /usr/src/linux/include/linux/cpumask.h:107
Process lscpu (pid: 575, task: 000000007deb4338, ksp: 000000007794f588)
Krnl PSW : 0704000180000000 0000000000106db4 (show_cpuinfo+0x108/0x234)
           R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:0 CC:0 PM:0 EA:3
Krnl GPRS: 0000000000000003 0000000000791988 000000000071b478 0000000000000004
           0000000000000001 0000000000000000 000000007d139500 0000000000000400
           0000000000000000 000000000070e24c 000000007d48d600 0000000000000005
           000000007d48d600 00000000004dfa10 0000000000106cf8 000000007794fcc0
Krnl Code: 0000000000106da8: 95001000           cli     0(%r1),0
           0000000000106dac: a774ffac           brc     7,106d04
           0000000000106db0: a7f40001           brc     15,106db2
          >0000000000106db4: 92011000           mvi     0(%r1),1
           0000000000106db8: a7f4ffa6           brc     15,106d04
           0000000000106dbc: c0e5000065b4       brasl   %r14,113924
           0000000000106dc2: c09000303a45       larl    %r9,70e24c
           0000000000106dc8: c020001eefd4       larl    %r2,4e4d70

Replacing NR_CPUS with nr_cpu_ids fixes it.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>