block/file-posix: Fix fully preallocated truncate

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>