mm/page_ext.c: constify lookup_page_ext() argument

mm/page_ext.c: constify lookup_page_ext() argument

lookup_page_ext() finds 'struct page_ext' for a given page.  It requires
only read access to the given struct page.

Current implemnentation takes 'struct page *' as an argument.  It makes
compiler complain when 'const struct page *' passed.

Change the argument to 'const struct page *'.

Link: http://lkml.kernel.org/r/20180531135457.20167-3-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>