xen-blkback: read from indirect descriptors only once

xen-blkback: read from indirect descriptors only once

Since indirect descriptors are in memory shared with the frontend, the
frontend could alter the first_sect and last_sect values after they have
been validated but before they are recorded in the request.  This may
result in I/O requests that overflow the foreign page, possibly
overwriting local pages when the I/O request is executed.

When parsing indirect descriptors, only read first_sect and last_sect
once.

This is part of XSA155.

CC: [email protected]
Signed-off-by: Roger Pau Monné <[email protected]>
Signed-off-by: David Vrabel <[email protected]>
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>