device-dax: fix pud fault fallback handling

Jeff Moyer reports:

    With a device dax alignment of 4KB or 2MB, I get sigbus when running
    the attached fio job file for the current kernel (4.11.0-rc1+).  If
    I specify an alignment of 1GB, it works.

    I turned on debug output, and saw that it was failing in the huge
    fault code.

     dax dax1.0: dax_open
     dax dax1.0: dax_mmap
     dax dax1.0: dax_dev_huge_fault: fio: write (0x7f08f0a00000 -
     dax dax1.0: __dax_dev_pud_fault: phys_to_pgoff(0xffffffffcf60)
     dax dax1.0: dax_release

    fio config for reproduce:
    [global]
    ioengine=dev-dax
    direct=0
    filename=/dev/dax0.0
    bs=2m

    [write]
    rw=write

    [read]
    stonewall
    rw=read

The driver fails to fallback when taking a fault that is larger than
the device alignment, or handling a larger fault when a smaller
mapping is already established. While we could support larger
mappings for a device with a smaller alignment, that change is
too large for the immediate fix. The simplest change is to force
fallback until the fault size matches the alignment.

Reported-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>