net/smc: replace mutex rmbs_lock and sndbufs_lock with rw_semaphore

It's clear that rmbs_lock and sndbufs_lock are aims to protect the
rmbs list or the sndbufs list.

During connection establieshment, smc_buf_get_slot() will always
be invoked, and it only performs read semantics in rmbs list and
sndbufs list.

Based on the above considerations, we replace mutex with rw_semaphore.
Only smc_buf_get_slot() use down_read() to allow smc_buf_get_slot()
run concurrently, other part use down_write() to keep exclusive
semantics.

Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>