net/core: Document all dev_ioctl() arguments

This patch avoids that the following warnings are reported when building
with W=1:

net/core/dev_ioctl.c:378: warning: Function parameter or member 'ifr' not described in 'dev_ioctl'
net/core/dev_ioctl.c:378: warning: Function parameter or member 'need_copyout' not described in 'dev_ioctl'
net/core/dev_ioctl.c:378: warning: Excess function parameter 'arg' description in 'dev_ioctl'

Cc: Al Viro <viro@zeniv.linux.org.uk>
Fixes: 44c02a2c

 ("dev_ioctl(): move copyin/copyout to callers") # v4.16.
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: David S. Miller <davem@davemloft.net>