arm64: mm: use enum to identify pgtable level instead of *_SHIFT

Commit 90292aca9854 ("arm64: mm: use appropriate ctors for page tables")
introduced pgtable ctor calls in pgd_pgtable_alloc().  To identify the
pgtable level and call the appropriate ctor, the *_SHIFT value associated
with the pgtable level is used.  However, those values do not
unambiguously identify a level, because if a given level is folded, the
*_SHIFT value will be equal to that of the upper level (e.g.  PMD_SHIFT ==
PUD_SHIFT if PMD is folded).

As things stand, there is probably not much damaged done by calling the
ctor for a different level, and ARCH_ENABLE_SPLIT_PMD_PTLOCK is only
selected if PMD isn't folded (so we don't needlessly initialise
pmd_ptlock).  Still, this is pretty confusing, and it would get even more
confusing when adding ctor calls for the remaining levels.

Let's simplify all this by using an enum to identify the pgtable level
instead; this way folding becomes irrelevant.  This is inspired by one of
the m68k pgtable allocators (arch/m68k/include/asm/motorola_pgalloc.h).

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kevin Brodsky <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Andreas Larsson <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Linus Waleij <[email protected]>
Cc: Madhavan Srinivasan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Qi Zheng <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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