o Since we always update either fragdrop or fragtimeout stat counter when we

free a fragment, provide two inline functions that do that for us:
ipq_drop() and ipq_timeout().

o Rename ip_free_f() to ipq_free() to match the name scheme of IP reassembly.
o Remove assertion from ipq_free(), since it requires extra argument to be

passed, but locking scheme is simple enough and function is static.

Sponsored by: Nginx, Inc.