async_xor: check src_offs is not NULL before updating it

When PAGE_SIZE is greater than 4kB, multiple stripes may share the same
page. Thus, src_offs is added to async_xor_offs() with array of offsets.
However, async_xor() passes NULL src_offs to async_xor_offs(). In such
case, src_offs should not be updated. Add a check before the update.

Fixes: ceaf2966

(async_xor: increase src_offs when dropping destination page)
Cc: stable@vger.kernel.org # v5.10+
Reported-by: Oleksandr Shchirskyi <oleksandr.shchirskyi@linux.intel.com>
Tested-by: Oleksandr Shchirskyi <oleksandr.shchirskyi@intel.com>
Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>