block/file-posix: Fix fully preallocated truncate
    
    Storing the lseek() result in an int results in it overflowing when the
    file is at least 2 GB big.  Then, we have a 50 % chance of the result
    being "negative" and thus thinking an error occurred when actually
    everything went just fine.
    
    So we should use the correct type for storing the result: off_t.
    
    Reported-by: Daniel P. Berrange <berrange@redhat.com>
    Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1549231
    Cc: qemu-stable@nongnu.org
    Signed-off-by: Max Reitz <mreitz@redhat.com>
    Message-id: 20180228131315.30194-2-mreitz@redhat.com
    Reviewed-by: Eric Blake <eblake@redhat.com>
    Signed-off-by: Max Reitz <mreitz@redhat.com>
    (cherry picked from commit 82b45e0a0b824787bd79ce3f6453eaa2afddd138)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>