udp: be less conservative with sock rmem accounting

Before commit 850cbaddb52d ("udp: use it's own memory accounting
schema"), the udp protocol allowed sk_rmem_alloc to grow beyond
the rcvbuf by the whole current packet's truesize. After said commit
we allow sk_rmem_alloc to exceed the rcvbuf only if the receive queue
is empty. As reported by Jesper this cause a performance regression
for some (small) values of rcvbuf.

This commit is intended to fix the regression restoring the old
handling of the rcvbuf limit.

Reported-by: Jesper Dangaard Brouer <[email protected]>
Fixes: 850cbaddb52d ("udp: use it's own memory accounting schema")
Signed-off-by: Paolo Abeni <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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