net/rds: initialize i_conn_path in rds_inc_init()

rds_inc_init() initializes every field of the embedded rds_incoming
except i_conn_path, and incomings are not zero-allocated (IB carves
them out of a slab cache).  The field therefore holds stale garbage
for incs created by rds_ib.

The loopback transport is different: rds_loop_xmit() re-runs
rds_inc_init() on the message's embedded inc after
rds_send_queue_rm() has already stored the connection path in it, so
there the field holds a live value rather than garbage, and a NULL
store would discard it.  Switch rds_loop_xmit() to
rds_inc_path_init() with the connection's single path, which is
exactly the value readers of the field reconstruct for a
non-multipath transport.

With loopback preserving the field, initialize it to NULL in
rds_inc_init() so that any future reader trips over a clean NULL
pointer instead of a stale one, and so the two init helpers
(rds_inc_init/rds_inc_path_init) leave the structure in an
equivalent, fully-initialized state.  Hardening only; no reader
dereferences i_conn_path for a non-multipath transport today.

This mirrors Oracle UEK commit "rds: rds_inc_init() should initialize
the inc->i_conn_path field".

Signed-off-by: William Kucharski <william.kucharski@oracle.com>
[achender: port to net-next; keep loopback's i_conn_path valid by
 switching rds_loop_xmit() to rds_inc_path_init(); update commit
 message]
Assisted-by: Claude-Code:claude-fable-5
Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260809005103.82371-3-achender@kernel.org
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>