ofproto-dpif-xlate: Generate bitmasks in set_field.

ofproto-dpif-xlate: Generate bitmasks in set_field.

Previously, whenever a set_field() action was executed, the entire field
would become masked and the entire field replaced, regardless of the
mask specified in the set_field() action.

In most cases this is fine, although it may lead to more specific
wildcards than strictly necessary. However, in a particular case with
connection tracking actions it could lead to the wrong behaviour.

Unlike most OpenFlow fields, the ct_{mark,labels} fields are typically
unknown until the ct(...,recirc_table=N,...) action is executed however
the packet may actually belong to a connection which has a nonzero value
for one of these fields. This can lead to the wrong behaviour with flows
such as the following:

in_port=1,ip,actions=ct(commit,exec(set_field(0x1/0x1->ct_mark))),2
in_port=2,ip,actions=ct(commit,exec(set_field(0x2/0x2->ct_mark))),1

Connections flowing through these actions will always update the ct_mark
field stored within the conntrack table. However, rather than modifying
only the specified bits (0x1 in one direction, 0x2 in the other), the
entire ct_mark field will be replaced. Such connections will constantly
toggle the value of ct_mark between 0x1 and 0x2, rather than becoming
0x3 and keeping that value.

This commit fixes the issue by ensuring that set_field actions only
modify the modified bits in the wildcards, rather than masking the
entire field.

Fixes: 8e53fe8cf7a1 ("Add connection tracking mark support.")
Fixes: 9daf23484fb1 ("Add connection tracking label support.")
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>