Merge branch 'rds-buffer-tuning'

Sowmini Varadhan says:

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RDS: TCP: tunable socket buffer parameters

Patch 1 uses sysctl to create tunable socket buffer size parameters.

Patch 2 removes an unuused constant.

v2: use sysctl
v3: review comments from Santosh Shilimkar, Eric Dumazet
v4: review comments from Hannes Sowa
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Signed-off-by: David S. Miller <davem@davemloft.net>