powerpc/pseries: Use H_CLEAR_HPT to clear MMU hash table during kexec

An hcall was recently added that does exactly what we need during kexec
- it clears the entire MMU hash table, ignoring any VRMA mappings.

Try it and fall back to the old method if we get a failure.

On a POWER8 box with 5TB of memory, this reduces the time it takes to
kexec a new kernel from from 4 minutes to 1 minute.

Signed-off-by: Anton Blanchard <[email protected]>
Tested-by: Mahesh Salgaonkar <[email protected]>
[mpe: Split into separate functions and tweak function naming]
Signed-off-by: Michael Ellerman <[email protected]>

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