xen/blkif: Avoid double access to src->nr_segments

src is stored in shared memory and src->nr_segments is dereferenced
twice at the end of the function.  If a compiler decides to compile this
into two separate memory accesses then the size limitation could be
bypassed.

Fix it by removing the double access to src->nr_segments.

This is part of XSA-155.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
(cherry picked from commit f9e98e5d)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>