The first page of the I/O virtual address space is reserved so that

The first page of the I/O virtual address space is reserved so that
is easier to spot misconfiguration or wrong behaviour where NULL is
used as address.  Right now that page is not part of the IOVA at all,
so when we reserve regions, like PCI I/O space, which can cover that
page as well, extent(9) will panic.  Instead, include it in the IOVA
but reserve it right away.  This way that page can be reserved twice.