net: fix sblock()

sblock() takes sb_rwlock for reading, and is the only such locker, so it
clearly has no effect.

Change it to acquire the lock for writing, so it serializes access to the
socket buffer as intended.

Bug introduced in 6296cbab.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>