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: NDaniel P. Berrange <berrange@redhat.com>
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1549231
Cc: qemu-stable@nongnu.org
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-id: 20180228131315.30194-2-mreitz@redhat.com
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/file-posix.c

@@ -1701,6 +1701,7 @@ static int raw_regular_truncate(int fd, int64_t offset, PreallocMode prealloc,
     case PREALLOC_MODE_FULL:
     {
         int64_t num = 0, left = offset - current_length;
+        off_t seek_result;
 
         /*
          * Knowing the final size from the beginning could allow the file
@@ -1715,8 +1716,8 @@ static int raw_regular_truncate(int fd, int64_t offset, PreallocMode prealloc,
 
         buf = g_malloc0(65536);
 
-        result = lseek(fd, current_length, SEEK_SET);
-        if (result < 0) {
+        seek_result = lseek(fd, current_length, SEEK_SET);
+        if (seek_result < 0) {
             result = -errno;
             error_setg_errno(errp, -result,
                              "Failed to seek to the old end of file");