pf: add hashtable row count SDT

This allows us to figure out how many states each hashrow contains. That
can be important to know when debugging performance issues.

A simple probe could be:

dtrace -n 'pf:purge:state:rowcount { @counts["states per row"] = quantize(arg1); }'
dtrace: description 'pf:purge:state:rowcount ' matched 1 probe
^C

	  states per row
	           value  ------------- Distribution ------------- count
	              -1 |                                         0
	               0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 8257624
	               1 |                                         14321
	               2 |                                         0

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