[ICMP]: Avoid sparse warnings in net/ipv4/icmp.c

   CHECK   net/ipv4/icmp.c
net/ipv4/icmp.c:249:13: warning: context imbalance in 'icmp_xmit_unlock' -
unexpected unlock
net/ipv4/icmp.c:376:13: warning: context imbalance in 'icmp_reply' - different
lock contexts for basic block
net/ipv4/icmp.c:430:6: warning: context imbalance in 'icmp_send' - different
lock contexts for basic block

Solution is to declare both icmp_xmit_lock() and icmp_xmit_unlock() as inline

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>