SUNRPC: Fix socket waits for write buffer space

[ Upstream commit 7496b59f588dd52886fdbac7633608097543a0a5 ]

The socket layer requires that we use the socket lock to protect changes
to the sock->sk_write_pending field and others.

Reported-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>