SUNRPC: two small improvements to rpcauth shrinker.

SUNRPC: two small improvements to rpcauth shrinker.

1/ If we find an entry that is too young to be pruned,
  return SHRINK_STOP to ensure we don't get called again.
  This is more correct, and avoids wasting a little CPU time.
  Prior to 3.12, it can prevent drop_slab() from spinning indefinitely.

2/ Return a precise number from rpcauth_cache_shrink_count(), rather than
  rounding down to a multiple of 100 (of whatever sysctl_vfs_cache_pressure is).
  This ensures that when we "echo 3 > /proc/sys/vm/drop_caches", this cache is
  still purged, even if it has fewer than 100 entires.

Neither of these are really important, they just make behaviour
more predicatable, which can be helpful when debugging related issues.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>