os/FileJournal: Tune the judge logic for read_header.

When reading journal-header, it should firstly check the result of
pread and then do decoce operation.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
os/FileJournal: Only using aio then alloc the related resources.

If define HAVE_LIBAIO, it alloc related resouces. But itt don't check whether
using aio mode. Only using aio it alloc the related resources.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
Merge pull request #2304 from majianpeng/fix1

small bug fix for journal

Reviewed-by: Sage Weil <sage@redhat.com>