open_and_lseek() increases the file size when the file is opened in
write mode and scrolled past the files end. This fails badly on /dev/mem
because loff_t which we use for the file size is signed variable, which
is used as an unsigned variable in /dev/mem. To catch this case do not
try to truncate FILE_SIZE_STREAM sized files.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>