Don't use 'int' to store lseek()'s return value to avoid problems with
large seek offsets. While at it, make sure to populate return error
code from 'errno'.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>