s390/bitops: fix find_next_bit_left

The find_next_bit_left function is broken if used with an offset which
is not a multiple of 64. The shift to mask the bits of a 64-bit word
not to search is in the wrong direction, the result can be either a
bit found smaller than the offset or failure to find a set bit.

Cc: <stable@vger.kernel.org> # v3.8+
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>