Always SSL_OP_NO_QUERY_MTU before querying the BIO MTU.

That setting means that the MTU is provided externally via SSL_set_mtu.

(Imported from upstream's 001235778a6e9c645dc0507cad6092d99c9af8f5)

Change-Id: I4e5743a9dee734ddd0235f080aefe98a7365aaf6
Reviewed-on: https://boringssl-review.googlesource.com/2826
Reviewed-by: Adam Langley <agl@google.com>