mm: fix process_vm_rw page counts

1. There is a "-1" missing in the page number calculation in
   process_vm_rw_core.  While this can't break anything, it can cause
   unnecessary allocations in certain cases:

   Consider handling an iovec ranging over PVM_MAX_PP_ARRAY_COUNT pages
   that is also aligned to a page boundary.  While pp_stack could hold
   references to such an amount of pinned pages, nr_pages yields
   (PVM_MAX_PP_ARRAY + 1) in process_vm_rw_core.  Consequently, a larger
   buffer is allocated with kmalloc for no reason.

   For any page boundary aligned iovec that is a multiple of PAGE_SIZE
   and larger than PVM_MAX_PP_ARRAY_COUNT pages, nr_pages will be too big
   by 1 and thus kmalloc allocates excess space for one more pointer.

2. max_pages_per_loop is constant and there is no reason to have it as
   a variable.  A macro does the job just fine and saves memory.

3. Replaced "sizeof(struct pages *)" with "sizeof(struct page *)" to
   have matching types for allocation and prevent confusion.

Link: https://lkml.kernel.org/r/20231111184859.44264-1-yjnworkstation@gmail.com
Signed-off-by: York Jasper Niebuhr <yjnworkstation@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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