sock: optimise sock_def_write_space barriers

Now we have a separate path for sock_def_write_space() and can go one
step further. When it's called from sock_wfree() we know that there is a
preceding atomic for putting down ->sk_wmem_alloc. We can use it to
replace to replace smb_mb() with a less expensive
smp_mb__after_atomic(). It also removes an extra RCU read lock/unlock as
a small bonus.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>