m68k: Implement generic_find_next_{zero_,}le_bit()

linux-next:
fs/udf/balloc.c: In function 'udf_bitmap_new_block':
fs/udf/balloc.c:274: error: implicit declaration of function 'generic_find_next_le_bit'

Convert ext2_find_next_{zero_,}bit() into generic_find_next_{zero_,}le_bit(),
and wrap the ext2_find_next_{zero_,}bit() around the latter.

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