riscv: mm: clarify ctor mm argument in alloc_{pte,pmd}_late

pagetable_{pte,pmd}_ctor(mm, ptdesc) skip the ptlock initialisation if mm
is &init_mm.  To avoid unnecessary overhead, it is therefore preferable to
pass the actual mm associated to the PTE/PMD.

Unfortunately, this proves challenging for alloc_{pte,pmd}_late() as the
associated mm is not available at the point where they are called - in
fact not even top-level functions like create_pgd_mapping() are passed the
mm.  As a result they both call the ctor with NULL as mm; this is safe but
potentially wasteful.

This is not a new situation, but let's add a couple of comments to clarify
it.

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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