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Normally we do an extra roundtrip for retries even if the buffer pool has
depleted, as we don't check that upfront. Rather than add this check, have
the buffer selection methods mark the request with REQ_F_BL_EMPTY if the
used buffer group is out of buffers after this selection. This is very
cheap to do once we're all the way inside there anyway, and it gives the
caller a chance to make better decisions on how to proceed.

For example, recv/recvmsg multishot could check this flag when it
decides whether to keep receiving or not.
Signed-off-by: Jens Axboe <axboe@kernel.dk>