m68k/bitops: Correct signature of test_bit()

mm/filemap.c: In function ‘clear_bit_unlock_is_negative_byte’:
mm/filemap.c:933: warning: passing argument 2 of ‘test_bit’ discards qualifiers from pointer target type

Make the bitmask pointed to by the "vaddr" parameter volatile to fix
this, like is done on other architectures.
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/bitops.h

@@ -148,7 +148,7 @@ static inline void bfchg_mem_change_bit(int nr, volatile unsigned long *vaddr)
 #define __change_bit(nr, vaddr)	change_bit(nr, vaddr)
 
 
-static inline int test_bit(int nr, const unsigned long *vaddr)
+static inline int test_bit(int nr, const volatile unsigned long *vaddr)
 {
 	return (vaddr[nr >> 5] & (1UL << (nr & 31))) != 0;
 }