docsrc: fix ifenslave type

Documentation/networking/ifenslave.c:1084: warning: pointer targets in assignment differ in signedness

>From include/linux/socket.h:
 *	1003.1g requires sa_family_t and that sa_data is char.

and from SUSv3:
(http://www.opengroup.org/onlinepubs/009695399/basedefs/sys/socket.h.html)

The <sys/socket.h> header shall define the sockaddr structure that includes at least the following members:

sa_family_t  sa_family  Address family.
char         sa_data[]  Socket address (variable-length data).
<end SUSv3>

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>