[S390] fix ext2_find_next_bit

ext4 does not work on s390 because ext2_find_next_bit is broken. Fortunately
this function is only used by ext4. The function uses ffs which does not work
analog to ffz. The result of ffs has an offset of 1 which is not taken into
account. To fix this use the low level __ffs_word function directly instead
of the ill defined ffs.

In addition the patch improves find_next_zero_bit and ext2_find_next_zero_bit
by passing the bit offset into __ffz_word instead of adding it after the
function call returned.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>