net/rds: rds_sendmsg should not discard payload_len

Commit 3db6e0d172c9 ("rds: use RCU to synchronize work-enqueue with
connection teardown") modifies rds_sendmsg to avoid enqueueing work
while a tear down is in progress. However, it also changed the return
value of rds_sendmsg to that of rds_send_xmit instead of the
payload_len. This means the user may incorrectly receive errno values
when it should have simply received a payload of 0 while the peer
attempts a reconnections. So this patch corrects the teardown handling
code to only use the out error path in that case, thus restoring the
original payload_len return value.

Fixes: 3db6e0d172c9 ("rds: use RCU to synchronize work-enqueue with connection teardown")
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260213035409.1963391-1-achender@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>