sunrpc: Add a sysfs file for one-step xprt deletion

Previously, the admin would need to set the xprt state to "offline"
before attempting to remove. This patch adds a new sysfs attr that does
both these steps in a single call.

Suggested-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>
Link: https://lore.kernel.org/r/20250207204225.594002-6-anna@kernel.org


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