Add lock to prevent a race with a file closing and also remove
useless and ugly sscanf code. The extra code was never needed
and the case it supposedly protected against is in fact handled
correctly by sock_from_file as pointed out by Al Viro.

CC: Neil Horman <nhorman@tuxdriver.com>
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>