file-posix: fix request_alignment typo

Fixes: a6b257a0
       ("file-posix: Handle undetectable alignment")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190827101328.4062-1-stefanha@redhat.com
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>