Debugging issues caused by pfmemalloc is often tedious.

Add a new SNMP counter to more easily diagnose these problems.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Josef Bacik <jbacik@fb.com>
Acked-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>