Make sockfs_setattr() static as it is not used outside of net/socket.c

This fixes the following GCC warning:
net/socket.c:534:5: warning: no previous prototype for ‘sockfs_setattr’ [-Wmissing-prototypes]

Fixes: 86741ec2 ("net: core: Add a UID field to struct sock.")
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: NTobias Klauser <tklauser@distanz.ch>
Acked-by: NLorenzo Colitti <lorenzo@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>