resize2fs: Add sanity check for off_t overflow before truncating

resize2fs: Add sanity check for off_t overflow before truncating

If we can't use ftruncate64(), and have to use ftruncate() instead,
make sure that we don't accidentally truncate the size when we chop it
down to an off_t before calling ftruncate(), lest we severely damage a
filesystem image file.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>