sctp: Fix COMM_LOST/CANT_STR_ASSOC err reporting on big-endian platforms

Commit 978aa04741 ("sctp: fix some type cast warnings introduced since
very beginning")' broke err reading from sctp_arg, because it reads the
value as 32-bit integer, although the value is stored as 16-bit integer.
Later this value is passed to the userspace in 16-bit variable, thus the
user always gets 0 on big-endian platforms. Fix it by reading the __u16
field of sctp_arg union, as reading err field would produce a sparse
warning.

Fixes: 978aa04741 ("sctp: fix some type cast warnings introduced since very beginning")
Signed-off-by: Petr Malat <oss@malat.biz>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Link: https://lore.kernel.org/r/20201030132633.7045-1-oss@malat.biz
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sctp/sm_sideeffect.c

@@ -1601,12 +1601,12 @@ static int sctp_cmd_interpreter(enum sctp_event_type event_type,
 			break;
 
 		case SCTP_CMD_INIT_FAILED:
-			sctp_cmd_init_failed(commands, asoc, cmd->obj.u32);
+			sctp_cmd_init_failed(commands, asoc, cmd->obj.u16);
 			break;
 
 		case SCTP_CMD_ASSOC_FAILED:
 			sctp_cmd_assoc_failed(commands, asoc, event_type,
-					      subtype, chunk, cmd->obj.u32);
+					      subtype, chunk, cmd->obj.u16);
 			break;
 
 		case SCTP_CMD_INIT_COUNTER_INC: