kernfs: implement custom exportfs ops and fid type

The current kernfs exportfs implementation uses the generic_fh_*()
helpers and FILEID_INO32_GEN[_PARENT] which limits ino to 32bits.
Let's implement custom exportfs operations and fid type to remove the
restriction.

* FILEID_KERNFS is a single u64 value whose content is
  kernfs_node->id.  This is the only native fid type.

* For backward compatibility with blk_log_action() path which exposes
  (ino,gen) pairs which userland assembles into FILEID_INO32_GEN keys,
  combine the generic keys into 64bit IDs in the same order.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>