smap: New macro SMAP_CONST1 for initializing immutable 1-member smaps.

smap: New macro SMAP_CONST1 for initializing immutable 1-member smaps.

Reviewing the ovn-controller code I started to notice a common pattern:

    struct smap ext_ids = SMAP_INITIALIZER(&ext_ids);
    smap_add(&ext_ids, "ovn-patch-port", network);
    ovsrec_port_set_external_ids(port, &ext_ids);
    smap_destroy(&ext_ids);

This seemed like a bit too much code for something as simple as
initializing an smap with a single key-value pair.  This commit allows the
code to be reduced to just:

    const struct smap ids = SMAP_CONST1(&ids, "ovn-patch-port", network);
    ovsrec_port_set_external_ids(port, &ids);

This new form also eliminates multiple memory allocation and free
operations, but I doubt that has any real effect on performance;
the primary goal here is code readability.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Russell Bryant <rbryant@redhat.com>