powerpc: Fix duplicate const clang warning in user access code

This re-applies commit b91c1e3e ("powerpc: Fix duplicate const
clang warning in user access code") (Jun 2015) which was undone in
commits:
  f2ca8090 ("powerpc/sparse: Constify the address pointer in __get_user_nosleep()") (Feb 2017)
  d466f6c5 ("powerpc/sparse: Constify the address pointer in __get_user_nocheck()") (Feb 2017)
  f84ed59a ("powerpc/sparse: Constify the address pointer in __get_user_check()") (Feb 2017)

We see a large number of duplicate const errors in the user access
code when building with llvm/clang:

  include/linux/pagemap.h:576:8: warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]
        ret = __get_user(c, uaddr);

The problem is we are doing const __typeof__(*(ptr)), which will hit
the warning if ptr is marked const.

Removing const does not seem to have any effect on GCC code
generation.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>