xfrm: snmp: do not use SNMP_MIB_SENTINEL anymore

Use ARRAY_SIZE(), so that we know the limit at compile time.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250905165813.1470708-9-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>