fs: key stableAttrs via a dedupKey indirection

Introduce mapIdentityTable.dedupKey, which for
externalNodeID=true dedupes based on StableAttr.Ino alone.

This lets the legacy code (node ID is autoincremented and always
unique, the current state) coexist with the new code
(ExternalNodeID=true: node ID is caller controlled and reusing a node
ID evicts from the kernel inode cache).

Change-Id: Ia08653d8603814b073e83790a01aa6026a6a6964