expression: cleanup expr_ops_by_type() and handle u32 input

expression: cleanup expr_ops_by_type() and handle u32 input

commit 3d0ce3c19d319a5aae806b617905cfa1ee7f87f4 upstream.

Make fewer assumptions about the underlying integer type of the enum.
Instead, be clear about where we have an untrusted uint32_t from netlink
and an enum. Rename expr_ops_by_type() to expr_ops_by_type_u32() to make
this clearer. Later we might make the enum as packed, when this starts
to matter more.

Also, only the code path expr_ops() wants strict validation and assert
against valid enum values. Move the assertion out of
__expr_ops_by_type(). Then expr_ops_by_type_u32() does not need to
duplicate the handling of EXPR_INVALID. We still need to duplicate the
check against EXPR_MAX, to ensure that the uint32_t value can be cast to
an enum value.

[ Remove cast on EXPR_MAX. --pablo ]

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>