Linux 4.6 compat: Fall back to d_prune_aliases() if necessary

As of 4.6, the icache and dcache LRUs are memcg aware insofar as the
kernel's per-superblock shrinker is concerned.  The effect is that dcache
or icache entries added by a task in a non-root memcg won't be scanned
by the shrinker in the context of the root (or NULL) memcg.  This defeats
the attempts by zfs_sb_prune() to unpin buffers and can allow metadata to
grow uncontrollably.  This patch reverts to the d_prune_aliaes() method
in case the kernel's per-superblock shrinker is not able to free anything.

Signed-off-by: Tim Chase <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Signed-off-by: Chunwei Chen <[email protected]>
Closes: #4726

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