In pmap_fault_fixup re-instate the TLB flush for the shared L1 case that
occurs for non-ARM_MMU_EXTENDED kernels.

This fixes rump/rumpkern/t_sp:stress_killer on rpi which is currently
non-ARM_MMU_EXTENDED
Remove an unnecessary flush that sneaked in as part of break-before-make
change.
Remove an XXXNH comment.