src: rework singleton interval transformation to reduce memory consumption

src: rework singleton interval transformation to reduce memory consumption

set_to_intervals() expands range expressions into a list of singleton
elements before building the netlink message that is sent to userspace.
This is because the kernel expects this list of singleton elements where
EXPR_F_INTERVAL_END denotes a closing interval. This expansion
significantly increases memory consumption in userspace.

This patch updates the logic to transform the range expression up to two
temporary singleton element expressions through setelem_to_interval().
Then, these two elements are used to allocate the nftnl_set_elem objects
through alloc_nftnl_setelem_interval() to build the netlink message,
finally all these temporary objects are released. For anonymous sets,
when adjacent ranges are found, the end element is not added to the set
to pack the set representation as in the original set_to_intervals()
routine.

After this update, set_to_intervals() only deals with adding the
non-matching all zero element to the interval set when it is not there
as the kernel expects.

In combination with the new EXPR_RANGE_VALUE expression, this shrinks
runtime userspace memory consumption from 70.50 Mbytes to 43.38 Mbytes
for a 100k intervals set sample.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>