Fix bug in the loop driver.

When presented with a multipage BIO, loop is overindexing the first
page in the BIO rather than advancing to the second page.  It scribbles
on the backing file and/or on kernel memory.

This happens with multipage BIO-based pagecache I/O and presumably with
O_DIRECT also.

The fix is much-needed with the multipage-BIO patches - using that code
on loop-backed filesystems has rather messy results.