bridge/mdb: fix missing new line when show bridge mdb

bridge/mdb: fix missing new line when show bridge mdb

The bridge mdb show is broken on current iproute2. e.g.
]# bridge mdb show
34: br0  veth0_br  224.1.1.2  temp 34: br0  veth0_br  224.1.1.1  temp

After fix:
]# bridge mdb show
34: br0  veth0_br  224.1.1.2  temp
34: br0  veth0_br  224.1.1.1  temp

v2: Use json print lib as Stephen suggested.
v3: No need to use is_json_context() as print_string() could handle both cases.
v4: use new function print_nl() to print new line in non-json mode.

Reported-by: Ying Xu <yinxu@redhat.com>
Fixes: c7c1a1ef51aea ("bridge: colorize output and use JSON print library")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>