sh: add copy_user_page() alias for __copy_user()

copy_user_page() is needed by DAX.  Without this we get a compile error
for DAX on SH:

  fs/dax.c:280:2: error: implicit declaration of function `copy_user_page' [-Werror=implicit-function-declaration]
    copy_user_page(vto, (void __force *)vfrom, vaddr, to);
      ^

This was done with a random config that happened to include DAX support.

This patch has only been compile tested.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Matthew Wilcox <willy@linux.intel.com>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>