RDMA/rtrs-clt: Check strnlen return len in sysfs mpath_policy_store()

strnlen() may return 0 (e.g. for "\0\n" string), it's better to
check the result of strnlen() before using 'len - 1' expression
for the 'buf' array index.

Detected using the static analysis tool - Svace.

Fixes: dc3b66a0

 ("RDMA/rtrs-clt: Add a minimum latency multipath policy")
Signed-off-by: Alexey Kodanev <aleksei.kodanev@bell-sw.com>
Link: https://lore.kernel.org/r/20240221113204.147478-1-aleksei.kodanev@bell-sw.com
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>