net: fix deadlock in net channel poll support

Path 1:

  poll()
   take file lock
   file::poll_install
     take socket lock

Path 2:

  sowakep() (holding socket lock)
    so_wake_poll()
      take file lock

Fix by running poll_install() outside the file lock (which isn't really
needed).

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>