powerpc/fixmap: Use __fix_to_virt() instead of fix_to_virt()

Modify back __set_fixmap() to using __fix_to_virt() instead
of fix_to_virt() otherwise the following happens because it
seems GCC doesn't see idx as a builtin const.

  CC      mm/early_ioremap.o
In file included from ./include/linux/kernel.h:11:0,
                 from mm/early_ioremap.c:11:
In function ‘fix_to_virt’,
    inlined from ‘__set_fixmap’ at ./arch/powerpc/include/asm/fixmap.h:87:2,
    inlined from ‘__early_ioremap’ at mm/early_ioremap.c:156:4:
./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_32’ declared with attribute error: BUILD_BUG_ON failed: idx >= __end_of_fixed_addresses
  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
                                      ^
./include/linux/compiler.h:331:4: note: in definition of macro ‘__compiletime_assert’
    prefix ## suffix();    \
    ^
./include/linux/compiler.h:350:2: note: in expansion of macro ‘_compiletime_assert’
  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
  ^
./include/linux/build_bug.h:39:37: note: in expansion of macro ‘compiletime_assert’
 #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
                                     ^
./include/linux/build_bug.h:50:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
  ^
./include/asm-generic/fixmap.h:32:2: note: in expansion of macro ‘BUILD_BUG_ON’
  BUILD_BUG_ON(idx >= __end_of_fixed_addresses);
  ^
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Fixes: 4cfac2f9 ("powerpc/mm: Simplify __set_fixmap()")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f4984c615f90caa3277775a68849afeea846850d.1568295907.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/fixmap.h

@@ -77,7 +77,12 @@ enum fixed_addresses {
 static inline void __set_fixmap(enum fixed_addresses idx,
 				phys_addr_t phys, pgprot_t flags)
 {
-	map_kernel_page(fix_to_virt(idx), phys, flags);
+	if (__builtin_constant_p(idx))
+		BUILD_BUG_ON(idx >= __end_of_fixed_addresses);
+	else if (WARN_ON(idx >= __end_of_fixed_addresses))
+		return;
+
+	map_kernel_page(__fix_to_virt(idx), phys, flags);
 }
 
 #endif /* !__ASSEMBLY__ */