riscv: Remove unnecessary invalidations in pmap_enter_quick_locked()

[markj]

Summary

This function always overwrites an invalid PTE, so if
pmap_try_insert_pv_entry() fails it is certainly not necessary to
invalidate anything, because the PTE has not yet been written by that
point.

It should also not be necessary to invalidate TLBs after overwriting an
invalid entry. In principle the TLB could cache negative entries, but
then the worst case scenario is a spurious fault. Since pmap_enter()
does not bother issuing an sfence.vma, pmap_enter_quick_locked() should
behave similarly.

[kib]

So RISC-V can cache -ve PTEs?

[kib]

sys/riscv/riscv/pmap.c

3463

BTW, return (NULL) there would be more clear

3476

And there

[jrtc27]

In D42291#965582, @kib wrote:

So RISC-V can cache -ve PTEs?

Yes, and the spec explicitly calls this out.

[markj]

Simplify some return paths.