Clean up reassignbuf() a bit.

[markj]

Summary

• Remove the reassignbufcalls sysctl. As the 20-year old comment above it indicates, it does not seem very useful. Also it is not precise and hurts scalability. getnewbufcalls was converted to counter(9), and we could do that here too, but the utility of the counter seems pretty dubious to begin with.
• Convert some panic() calls into KASSERTs. The pctrie code will panic if it fails to locate the specified key during deletion, so we get some protection if the buf is not present.

[mjg]

Side note: I think this should go in as 2 commits.

[kib]

sys/kern/vfs_subr.c

2720

BTW this sentence is false for quite long time.

[markj]

In D25965#575595, @mjg wrote:

Side note: I think this should go in as 2 commits.

Will do.

sys/kern/vfs_subr.c

2720

Both sentences, I guess.

[kib]

sys/kern/vfs_subr.c

2720

Yes, it is used only to reassign between queues now.

[markj]

Rewrite the reassignbuf() description.