net/smc: change SMCR_RMBE_SIZES from 5 to 15

SMCR_RMBE_SIZES is the upper boundary of SMC-R's snd_buf and rcv_buf.
The maximum bytes of snd_buf and rcv_buf can be calculated by 2^SMCR_
RMBE_SIZES * 16KB. SMCR_RMBE_SIZES = 5 means the upper boundary is 512KB.
TCP's snd_buf and rcv_buf max size is configured by net.ipv4.tcp_w/rmem[2]
whose default value is 4MB or 6MB, is much larger than SMC-R's upper
boundary.

In some scenarios, such as Recommendation System, the communication
pattern is mainly large size send/recv, where the size of snd_buf and
rcv_buf greatly affects performance. Due to the upper boundary
disadvantage, SMC-R performs poor than TCP in those scenarios. So it
is time to enlarge the upper boundary size of SMC-R's snd_buf and rcv_buf,
so that the SMC-R's snd_buf and rcv_buf can be configured to larger size
for performance gain in such scenarios.

The SMC-R rcv_buf's size will be transferred to peer by the field
rmbe_size in clc accept and confirm message. The length of the field
rmbe_size is four bits, which means the maximum value of SMCR_RMBE_SIZES
is 15. In case of frequently adjusting the value of SMCR_RMBE_SIZES
in different scenarios, set the value of SMCR_RMBE_SIZES to the maximum
value 15, which means the upper boundary of SMC-R's snd_buf and rcv_buf
is 512MB. As the real memory usage is determined by the value of
net.smc.w/rmem, not by the upper boundary, set the value of SMCR_RMBE_SIZES
to the maximum value has no side affects.

Signed-off-by: Guangguan Wang <[email protected]>
Co-developed-by: Wen Gu <[email protected]>
Signed-off-by: Wen Gu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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