powerpc: Always inline radix_enabled() to fix build failure

This is the same as commit acdad8fb ("powerpc: Force inlining of
mmu_has_feature to fix build failure") but for radix_enabled().  The
config in the linked bugzilla causes the following build failure:

  LD      .tmp_vmlinux.kallsyms1
  powerpc64-linux-ld: arch/powerpc/mm/pgtable.o: in function `.__ptep_set_access_flags':
  pgtable.c:(.text+0x17c): undefined reference to `.radix__ptep_set_access_flags'
  powerpc64-linux-ld: arch/powerpc/mm/pageattr.o: in function `.change_page_attr':
  pageattr.c:(.text+0xc0): undefined reference to `.radix__flush_tlb_kernel_range'
  etc.

This is due to radix_enabled() not being inlined. See extract from
building with -Winline:

  In file included from arch/powerpc/include/asm/lppaca.h:46,
                   from arch/powerpc/include/asm/paca.h:17,
                   from arch/powerpc/include/asm/current.h:13,
                   from include/linux/thread_info.h:23,
                   from include/asm-generic/preempt.h:5,
                   from ./arch/powerpc/include/generated/asm/preempt.h:1,
                   from include/linux/preempt.h:78,
                   from include/linux/spinlock.h:51,
                   from include/linux/mmzone.h:8,
                   from include/linux/gfp.h:6,
                   from arch/powerpc/mm/pgtable.c:21:
  arch/powerpc/include/asm/book3s/64/pgtable.h: In function '__ptep_set_access_flags':
  arch/powerpc/include/asm/mmu.h:327:20: error: inlining failed in call to 'radix_enabled': call is unlikely and code size would grow [-Werror=inline]

The code relies on constant folding of MMU_FTRS_POSSIBLE at buildtime
and elimination of non possible parts of code at compile time. For this
to work radix_enabled() must be inlined so make it __always_inline.
Reported-by: Erhard F. <erhard_f@mailbox.org>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
[mpe: Trimmed error messages in change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=213803
Link: https://lore.kernel.org/r/20210804013724.514468-1-jniethe5@gmail.com

arch/powerpc/include/asm/mmu.h

@@ -324,7 +324,7 @@ static inline void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
 }
 #endif /* !CONFIG_DEBUG_VM */
 
-static inline bool radix_enabled(void)
+static __always_inline bool radix_enabled(void)
 {
 	return mmu_has_feature(MMU_FTR_TYPE_RADIX);
 }