sunrpc: Add a sysfs attr for xprtsec

This allows the admin to check the TLS configuration for each xprt.

Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>
Link: https://lore.kernel.org/r/20250207204225.594002-3-anna@kernel.org


Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>