Revise SPL wrapper for shrinker callbacks

The SPL provides a wrapper for the kernel's shrinker callbacks, which
enables the ZFS code to interface with multiple versions of the shrinker
API's from different kernel versions.  Specifically, Linux kernels 3.0 -
3.11 has a single "combined" callback, and Linux kernels 3.12 and later
have two "split" callbacks.  The SPL provides a wrapper function so that
the ZFS code only needs to implement one version of the callbacks.

Currently the SPL's wrappers are designed such that the ZFS code
implements the older, "combined" callback.  There are a few downsides to
this approach:

* The general design within ZFS is for the latest Linux kernel to be
considered the "first class" API.

* The newer, "split" callback API is easier to understand, because each
callback has one purpose.

* The current wrappers do not completely abstract out the differing
API's, so ZFS code needs `#ifdef` code to handle the differing return
values required for different kernel versions.

This commit addresses these drawbacks by having the ZFS code provide the
latest, "split" callbacks, and the SPL provides a wrapping function for
the older, "combined" API.

Reviewed-by: Pavel Zakharov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #10502 

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