When initial placement of the new entry crosses the boundary,
allocator tries to move the entry up, after the boundary. The new
location may still fail to satisfy boundary requirement, for instance,
if the boundary is set to page size, and allocation is of multiple
pages.

Recheck that boundary is not crossed after the move. If it is
crossed, give up on allocating the whole entry and split it.

Reported by: Michael Fuckner <michael@fuckner.net>, running nvme(4)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week