NFSD: Rewrite synopsis of nfsd_percpu_counters_init()

In function ‘export_stats_init’,
    inlined from ‘svc_export_alloc’ at fs/nfsd/export.c:866:6:
fs/nfsd/export.c:337:16: warning: ‘nfsd_percpu_counters_init’ accessing 40 bytes in a region of size 0 [-Wstringop-overflow=]
  337 |         return nfsd_percpu_counters_init(&stats->counter, EXP_STATS_COUNTERS_NUM);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fs/nfsd/export.c:337:16: note: referencing argument 1 of type ‘struct percpu_counter[0]’
fs/nfsd/stats.h: In function ‘svc_export_alloc’:
fs/nfsd/stats.h:40:5: note: in a call to function ‘nfsd_percpu_counters_init’
   40 | int nfsd_percpu_counters_init(struct percpu_counter counters[], int num);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>