nx-match: Add context argument to nxm_put__().

An upcoming commit will need to pass an extra piece of data from
nx_put_raw() into all of its direct and indirect calls to nxm_put__().
This commit prepares for that by switching from a "struct ofpbuf *"
parameter to a context structure that, currently, contains just a
struct ofpbuf *.  The upcoming commit will add another member to the
context struct.

This commit has no visible effect on behavior.

Signed-off-by: Ben Pfaff <[email protected]>