None of the callers of write_full() expect a zero return value. Given
how the documentation explicitly states that either all of the buffer
is going to be written out or an error generated, treat 0 retrun from
write() as a error, set errno to ENOSPC and return -1.

Same logic applies to pwrite_full() as well, so make the change there
while at it.

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