net: do not read sk_drops if application does not care

net: do not read sk_drops if application does not care

sk_drops can be an often written field, do not read it unless
application showed interest.

Note that sk_drops can be read via inet_diag, so applications
can avoid getting this info from every received packet.

In the future, 'reading' sk_drops might require folding per node or per
cpu fields, and thus become even more expensive than today.

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Paolo Abeni <[email protected]>
Signed-off-by: David S. Miller <[email protected]>