tls: Rename TLS_INFO_ZC_SENDFILE to TLS_INFO_ZC_TX

tls: Rename TLS_INFO_ZC_SENDFILE to TLS_INFO_ZC_TX

To embrace possible future optimizations of TLS, rename zerocopy
sendfile definitions to more generic ones:

* setsockopt: TLS_TX_ZEROCOPY_SENDFILE- > TLS_TX_ZEROCOPY_RO
* sock_diag: TLS_INFO_ZC_SENDFILE -> TLS_INFO_ZC_RO_TX

RO stands for readonly and emphasizes that the application shouldn't
modify the data being transmitted with zerocopy to avoid potential
disconnection.

Fixes: c1318b39c7d3 ("tls: Add opt-in zerocopy mode of sendfile()")
Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Link: https://lore.kernel.org/r/20220608153425.3151146-1-maximmi@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>