mm: move vma_shrink(), vma_expand() to internal header

The vma_shrink() and vma_expand() functions are internal VMA manipulation
functions which we ought to abstract for use outside of memory management
code.

To achieve this, we replace shift_arg_pages() in fs/exec.c with an
invocation of a new relocate_vma_down() function implemented in mm/mmap.c,
which enables us to also move move_page_tables() and vma_iter_prev_range()
to internal.h.

The purpose of doing this is to isolate key VMA manipulation functions in
order that we can both abstract them and later render them easily
testable.

Link: https://lkml.kernel.org/r/3cfcd9ec433e032a85f636fdc0d7d98fafbd19c5.1722251717.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Brendan Higgins <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: David Gow <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Rae Moar <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Pengfei Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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