[fdbshow]: Handle FDB cleanup gracefully. (sonic-net#1926)

The race condition is caused by a blocking Redis call which gets the contents of the FDB entry from ASIC DB.
Since it has been implemented as a simple loop, there is no guarantee that entry will be present in DB when the contents are being read.

- What I did
Fixed: [fdb] 'show mac' command failed with t0-56-po2vlan topology sonic-net#1866

- How I did it
Removed blocking calls from fdbshow

- How to verify it
Run FDB test

- Previous command output (if the output of a command-line utility has changed)
root@sonic:/home/admin# show mac
Key 'ASIC_STATE:SAI_OBJECT_TYPE_FDB_ENTRY:{"bvid":"oid:0x260000000009cc","mac":"02:11:22:33:20:00","switch_id":"oid:0x21000000000000"}' unavailable in database '1'

- New command output (if the output of a command-line utility has changed)
root@sonic:/home/admin# show mac
No.    Vlan    MacAddress    Port    Type
-----  ------  ------------  ------  ------
Total number of entries 0

Signed-off-by: Nazarii Hnydyn <nazariig@nvidia.com>

scripts/fdbshow

@@ -83,7 +83,7 @@ class FdbShow(object):
         if not self.if_br_oid_map:
             return
 
-        fdb_str = self.db.keys('ASIC_DB', "ASIC_STATE:SAI_OBJECT_TYPE_FDB_ENTRY:*")
+        fdb_str = self.db.keys(self.db.ASIC_DB, "ASIC_STATE:SAI_OBJECT_TYPE_FDB_ENTRY:*")
         if not fdb_str:
             return
 
@@ -95,7 +95,10 @@ class FdbShow(object):
             if not fdb:
                 continue
 
-            ent = self.db.get_all('ASIC_DB', s, blocking=True)
+            ent = self.db.get_all(self.db.ASIC_DB, s)
+            if not ent:
+                continue
+
             br_port_id = ent["SAI_FDB_ENTRY_ATTR_BRIDGE_PORT_ID"][oid_pfx:]
             ent_type = ent["SAI_FDB_ENTRY_ATTR_TYPE"]
             fdb_type = ['Dynamic','Static'][ent_type == "SAI_FDB_ENTRY_TYPE_STATIC"]