Fixed off-by-one check for the DMAP limit.

Should now be fixed. I misinterpreted the desired behaviour for PHYS_SZ_IN_DMAP.

Added the PHYS_SZ_IN_DMAP macro as requested.

@kib Do you have any further comment?

In D40110#916824, @mhorne wrote:

LGTM, thanks. I can push this to the main branch.

Can you update the title+summary to reflect the final diff? (We didn't end up changing the devmap size.) Even easier would be if you could just send the git format-patch output to me over email: mhorne@freebsd.org.

Shrink the size of PMAP_MAPDEV_EARLY_SIZE to a single L2_SIZE.
This should be acceptable given the fact that it does not seem that we were
trampling the DTB mapping before.
While this solution is less robust, it avoids adding complexity to asm code
unless it is absolutely required.
Added assertions that verify that both the DTB and early DEVMAP are aligned to
L2 boundary and that the DTB mapping does not overlap the DEVMAP.
This check assumes that the DTB only consumes a single L2 entry.

@mhorne I'm not sure how this currently is supposed to work, because the PMAP_MAPDEV_EARLY_SIZE constant is already larger than L2_SIZE in vmparam.h. So what we really want is also the DTB to be mapped at VM_EARLY_DTB_ADDRESS, which currently seems to be assumed to correspond to the second-to-last L2 entry in the devmap table. Is the PMAP_MAPDEV_EARLY_SIZE constant supposed to contain VM_EARLY_DTB_ADDRESS?

Looks good to me.