sunrpc: Add a sysfs file for adding a new xprt

Writing to this file will clone the 'main' xprt of an xprt_switch and
add it to be used as an additional connection.

--

Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>
v3: Replace call to xprt_iter_get_xprt() with xprt_iter_get_next()
Link: https://lore.kernel.org/r/20250207204225.594002-5-anna@kernel.org


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