NLM: Remove AF_UNSPEC arm in nlm_display_address()

AF_UNSPEC support is no longer needed in nlm_display_address() now
that a presentation address is no longer generated for the h_srcaddr
field.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>