exportfs: Add a common exit label in exportfs()

Clean up: Reduce code duplication by introducing a goto label for
freeing hp and exiting.  This will make replacing "struct hostent *"
with "struct addrinfo *" more straightforward in this code.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>