Patch series "Add support for fast mremap"/

This series speeds up the mremap(2) syscall by copying page tables at the
PMD level even for non-THP systems.  There is concern that the extra
'address' argument that mremap passes to pte_alloc may do something subtle
architecture related in the future that may make the scheme not work.
Also we find that there is no point in passing the 'address' to pte_alloc
since its unused.  This patch therefore removes this argument tree-wide
resulting in a nice negative diff as well.  Also ensuring along the way
that the enabled architectures do not do anything funky with the 'address'
argument that goes unnoticed by the optimization.

Build and boot tested on x86-64.  Build tested on arm64.  The config
enablement patch for arm64 will be posted in the future after more
testing.

The changes were obtained by applying the following Coccinelle script.
(thanks Julia for answering all Coccinelle questions!).
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