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: Geert Uytterhoeven <geert@linux-m68k.org>