datapath: Add support for unique flow identifiers.

Previously, flows were manipulated by userspace specifying a full,
unmasked flow key. This adds significant burden onto flow
serialization/deserialization, particularly when dumping flows.

This patch adds an alternative way to refer to flows using a
variable-length "unique flow identifier" (UFID). At flow setup time,
userspace may specify a UFID for a flow, which is stored with the flow
and inserted into a separate table for lookup, in addition to the
standard flow table. Flows created using a UFID must be fetched or
deleted using the UFID.

All flow dump operations may now be made more terse with OVS_UFID_F_*
flags. For example, the OVS_UFID_F_OMIT_KEY flag allows responses to
omit the flow key from a datapath operation if the flow has a
corresponding UFID. This significantly reduces the time spent assembling
and transacting netlink messages. With all OVS_UFID_F_OMIT_* flags
enabled, the datapath only returns the UFID and statistics for each flow
during flow dump, increasing ovs-vswitchd revalidator performance by up
to 50%.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
CC: netdev@vger.kernel.org
---
v11: Separate UFID and unmasked key from sw_flow.
     Modify interface to remove nested UFID attributes.
     Perform complete nlmsg_parsing in ovs_flow_cmd_dump().
     Move UFID nla fetch helpers to flow_netlink.h.
     Check UFID table for flows with duplicate UFID at flow setup.
     Tidy up mask/key/ufid insertion into flow_table
     Rebase.
v10: Ignore flow_key in requests if UFID is specified.
     Only allow UFID flows to be indexed by UFID.
     Only allow non-UFID flows to be indexed by unmasked flow key.
     Unite the unmasked_key and ufid+ufid_hash in 'struct sw_flow'.
     Don't periodically rehash the UFID table.
     Resize the UFID table independently from the flow table.
     Modify table_destroy() to iterate once and delete from both tables.
     Fix UFID memory leak in flow_free().
     Remove kernel-only UFIDs for non-UFID cases.
     Rename "OVS_UFID_F_SKIP_*" -> "OVS_UFID_F_OMIT_*"
     Update documentation.
     Rebase.
v9: No change.
v8: Rename UID -> UFID "unique flow identifier".
    Fix null dereference when adding flow without uid or mask.
    If UFID and not match are specified, and lookup fails, return ENOENT.
    Rebase.
v7: Remove OVS_DP_F_INDEX_BY_UID.
    Rework UID serialisation for variable-length UID.
    Log error if uid not specified and OVS_UID_F_SKIP_KEY is set.
    Rebase against "probe" logging changes.
v6: Fix documentation for supporting UIDs between 32-128 bits.
    Minor style fixes.
    Rebase.
v5: No change.
v4: Fix memory leaks.
    Log when triggering the older userspace issue above.
v3: Initial post.