net/mlx5: DR, Improve steering for empty or RX/TX-only matchers

Every matcher has RX and TX paths. When a new matcher is created, its RX
and TX start/end anchors are connected to the respective RX and TX anchors
of the previous and next matchers.
This creates a potential performance issue: when a certain rule is added
to a matcher, in many cases it is RX or TX only rule, which may create a
long chain of RX/TX-only paths w/o the actual rules.

This patch aims to handle this issue.

RX and TX matchers are now handled separately: matcher connection in the
matchers chain is split into two separate lists: RX only and TX only.
when a new matcher is created, it is initially created 'detached' - its
RX/TX members are not inserted into the table's matcher list.
When an actual rule is added, only its appropriate RX or TX nic matchers
are then added to the table's nic matchers list and inserted into its
place in the chain of matchers.
I.e., if the rule that is being added is an RX-only rule, only the RX
part of the matcher will be connected to the chain, while TX part of the
matcher remains detached and doesn't prolong the TX chain of the matchers.

Same goes for rule deletion: when the last RX/TX rule of the nic matcher
is destroyed, the nic matcher is removed from its list.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>