ss: Shorter display format for TLS zerocopy sendfile

ss: Shorter display format for TLS zerocopy sendfile

Commit 21c07b45688f ("ss: Show zerocopy sendfile status of TLS
sockets") started displaying the activation status of zerocopy sendfile
on TLS sockets, exposed via sock_diag. This commit makes the format more
compact: the flag's name is shorter and is printed only when the feature
is active, similar to other flag options.

The flag's name is also generalized ("sendfile" -> "tx") to embrace
possible future optimizations, and includes an explicit indication that
the underlying data must not be modified during transfer ("ro").

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>