sock: dedup sock_def_write_space wmem_alloc checks

Except for minor rounding differences the first ->sk_wmem_alloc test in
sock_def_write_space() is a hand coded version of sock_writeable().
Replace it with the helper, and also kill the following if duplicating
the check.

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