x86 busdma: fix mis-use of bus_addr_t where vm_paddr_t is assumed.

Right now bus_addr_t and vm_paddr_t are always aliased to the same
underlying integer type on x86, which makes the interchange hard to
detect. Shortly, i386 kernel would use uint64_t for vm_paddr_t to
enable automatic use of PAE paging structures if hardware allows it,
while bus_addr_t would be extended to 64bit only when PAE option is
specified.

Fix all places that were identified as using bus_addr_t while page
address was assumed. This was performed by testing the complete PAE
merging patch on machine with > 4G of RAM enabled.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D18854