Set PTE_A and PTE_D for user mappings in pmap_enter().

[jhb]

Summary

This assumes that an access according to the prot in 'flags' triggered
a fault and is going to be retried after the fault returns, so the two
flags are set preemptively to avoid refaulting on the retry.

While here, only bother setting PTE_D for kernel mappings in pmap_enter
for writable mappings.

Test Plan

• booted qemu and ran various programs

[markj]

sys/riscv/riscv/pmap.c

2093 ↗

(On Diff #49840)

Should be new_l3, as you corrected in D17783.