netfs: zero gaps in read-gaps folio to avoid writing back stale data

In the Linux kernel, the following vulnerability has been resolved:

netfs: zero gaps in read-gaps folio to avoid writing back stale data

netfs_read_gaps() leaves gaps around a streaming write's dirty region
unread on the server side, so a short/EOF response there left stale
folio content that later got written back.

Zero it after the read completes, not before: netfs_wait_for_read()
returns rreq->transferred once ret >= 0, so build an iterator over the
same bvec array the read used, advance past ret, and zero the rest.
The dirty region already maps to sink pages in that array rather than
the real folio, so this can't touch it, and only the actual shortfall
gets zeroed instead of the whole gap upfront.