m68k: Add braces to __pmd(x) initializer to kill compiler warning

m68k: Add braces to __pmd(x) initializer to kill compiler warning

With gcc 4.1.2:

    include/linux/swapops.h: In function ‘swp_entry_to_pmd’:
    include/linux/swapops.h:294: warning: missing braces around initializer
    include/linux/swapops.h:294: warning: (near initialization for ‘(anonymous).pmd’)

Due to a GCC zero initializer bug (#53119), the standard "(pmd_t){ 0 }"
initializer is not accepted by all GCC versions.
In addition, on m68k pmd_t is an array instead of a single value, so we
need "(pmd_t){ { 0 }, }" instead of "(pmd_t){ 0 }".

Based on commit 9157259d16a8 ("mm: add pmd_t initializer __pmd() to
work around a GCC bug.") for sparc32.

Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>