tcp: introduce tcp_under_memory_pressure()

tcp: introduce tcp_under_memory_pressure()

Introduce an optimized version of sk_under_memory_pressure()
for TCP. Our intent is to use it in fast paths.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(adapted from v4.9.x commit b8da51ebb1aa93908350f95efae73aecbc2e266c)

Orabug: 28639707
CVE: CVE-2018-5390

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>