rdma: Fix for ineffective check in add_filter()

rdma: Fix for ineffective check in add_filter()

With 'name' field defined as array in struct filters, it will always
contain a value irrespective of whether a name was assigned or not.

Fix this by turning the field into a const char pointer.

Fixes: 1174be72d1b4c ("rdma: Add filtering infrastructure")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>