[X25]: Avoid divides and sparse warnings

   CHECK   net/x25/af_x25.c
net/x25/af_x25.c:117:46: warning: expensive signed divide
   CHECK   net/x25/x25_facilities.c
net/x25/x25_facilities.c:209:30: warning: expensive signed divide
   CHECK   net/x25/x25_in.c
net/x25/x25_in.c:250:26: warning: expensive signed divide
   CHECK   net/x25/x25_proc.c
net/x25/x25_proc.c:48:11: warning: context imbalance in 'x25_seq_route_start'
- wrong count at exit
net/x25/x25_proc.c:72:13: warning: context imbalance in 'x25_seq_route_stop' -
unexpected unlock
net/x25/x25_proc.c:112:11: warning: context imbalance in
'x25_seq_socket_start' - wrong count at exit
net/x25/x25_proc.c:129:13: warning: context imbalance in 'x25_seq_socket_stop'
- unexpected unlock
net/x25/x25_proc.c:190:11: warning: context imbalance in
'x25_seq_forward_start' - wrong count at exit
net/x25/x25_proc.c:215:13: warning: context imbalance in
'x25_seq_forward_stop' - unexpected unlock
   CHECK   net/x25/x25_subr.c
net/x25/x25_subr.c:362:57: warning: expensiv...