sock: Correct error checking condition for (assign|release)_proto_idx()

sock: Correct error checking condition for (assign|release)_proto_idx()

(assign|release)_proto_idx() wrongly check find_first_zero_bit() failure
by condition '(prot->inuse_idx == PROTO_INUSE_NR - 1)' obviously.

Fix by correcting the condition to '(prot->inuse_idx == PROTO_INUSE_NR)'

Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250410-fix_net-v2-1-d69e7c5739a4@quicinc.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>