3.12 compat, NUMA-aware per-superblock shrinker

Kernels >= 3.12 have a NUMA-aware superblock shrinker which is used in
ZoL by zfs_sb_prune(). This patch calls the shrinker for each on-line
NUMA node in order that memory be freed for each one.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3495