nhrpd: fix sending /32 shortcut

nhrpd: fix sending /32 shortcut

The remote spoke always sends a 32 prefix length to a shortcut request.
In the example, the remote spoke as the IP address 192.168.2.1/24.

spoke1# sh ip nhrp shortcut
Type     Prefix                   Via                      Identity
dynamic  192.168.2.1/32           10.255.255.2

Do not deal with local routes in nhrpd. Now:

spoke1# sh ip nhrp shortcut
Type     Prefix                   Via                      Identity
dynamic  192.168.2.0/24           10.255.255.2

Fixes: d4aa24ba7d ("*: Introduce Local Host Routes to FRR")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>