Fix return value checking of fread() in iolog.c

Fix return value checking of fread() in iolog.c

According to http://pubs.opengroup.org/onlinepubs/7908799/xsh/fread.html
fread() returns a size_t, not ssize_t, and returns a value of 0 on
both eof and an error. Therefore, check both feof() and ferror() to
determine whether the call succeeded.

Signed-off-by: Jens Axboe <axboe@kernel.dk>