The USE_MMAP code can fail, and the caller handles the failure
already.  Let the !USE_MMAP code fail as well, for consistency.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NGonglei <arei.gonglei@huawei.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>