sbin/fsck_hammer2: Add -c to cache subtree blockref stats for speed

sbin/fsck_hammer2: Add -c to cache subtree blockref stats for speed

If blockref scan succeeds and subtree blockref count exceeds a -c
value, cache subtree blockref stats and blockref itself.

This greatly improves speed when multiple PFSes share large parts
of blockrefs via snapshots. If blockrefs were somehow corrupted in
a weird way that exact same copies of blockref appear more than once,
this doesn't work, but should generally be useful.

Note that using a too small -c value may eat memory without much
performance improvement. Also note that using a too large -c value
saves memory but blockrefs may not be cached or used.

A suitable -c value depends on blockref tree structure since it's
a threshold for blockref count. A fs with more than a few GB of
consumption, should use a few thousand or more.