src: replace struct stmt_ops by type field in struct stmt

src: replace struct stmt_ops by type field in struct stmt

Shrink struct stmt in 8 bytes.

__stmt_ops_by_type() provides an operation for STMT_INVALID since this
is required by -o/--optimize.

There are many checks for stmt->ops->type, which is the most accessed
field, that can be trivially replaced.

BUG() uses statement type enum instead of name.

Similar to:

 68e76238749f ("src: expr: add and use expr_name helper").
 72931553828a ("src: expr: add expression etype")
 2cc91e6198e7 ("src: expr: add and use internal expr_ops helper")

Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>