RDMA/iwpm: fix memory leak on map_info

In the cases where iwpm_hash_bucket is NULL and where function
get_mapinfo_hash_bucket returns NULL then the map_info is never added
to hash_bucket_head and hence there is a leak of map_info. Fix this
by nullifying hash_bucket_head and if that is null we know that
that map_info was not added to hash_bucket_head and hence map_info
should be free'd.

Detected by CoverityScan, CID#1222481 ("Resource Leak")

Fixes: 30dc5e63

 ("RDMA/core: Add support for iWARP Port Mapper user space service")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>