Do not enter vendor SAI critical section for counter polling/clearing operations (sonic-net#1450)

There are two types of threads in syncd:

The syncd main thread in which the create/destroy/set/get SAI APIs are called
Flex counter threads in which counter-polling/-clearing APIs are called
The critical section in vendor SAI was introduced to protect vendors' SAI from being re-entered, which prevents the flex counter threads from running concurrently, and introduces latency for one flex counter thread when it's waiting for the critical section.
It is not necessary to enter a section in counter-polling/clearing operations since the objects' state won't be changed in that API.

Author: stephenxs

syncd/VendorSai.cpp

@@ -313,7 +313,6 @@ sai_status_t VendorSai::getStats(
         _In_ const sai_stat_id_t *counter_ids,
         _Out_ uint64_t *counters)
 {
-    MUTEX();
     SWSS_LOG_ENTER();
     VENDOR_CHECK_API_INITIALIZED();
 
@@ -351,7 +350,6 @@ sai_status_t VendorSai::getStatsExt(
         _In_ sai_stats_mode_t mode,
         _Out_ uint64_t *counters)
 {
-    MUTEX();
     SWSS_LOG_ENTER();
     VENDOR_CHECK_API_INITIALIZED();
 
@@ -366,7 +364,6 @@ sai_status_t VendorSai::clearStats(
         _In_ uint32_t number_of_counters,
         _In_ const sai_stat_id_t *counter_ids)
 {
-    MUTEX();
     SWSS_LOG_ENTER();
     VENDOR_CHECK_API_INITIALIZED();
 
@@ -386,7 +383,6 @@ sai_status_t VendorSai::bulkGetStats(
         _Inout_ sai_status_t *object_statuses,
         _Out_ uint64_t *counters)
 {
-    MUTEX();
     SWSS_LOG_ENTER();
     VENDOR_CHECK_API_INITIALIZED();
 
@@ -414,7 +410,6 @@ sai_status_t VendorSai::bulkClearStats(
         _In_ sai_stats_mode_t mode,
         _Inout_ sai_status_t *object_statuses)
 {
-    MUTEX();
     SWSS_LOG_ENTER();
     VENDOR_CHECK_API_INITIALIZED();