dax: Fix alloc_dax_region() compile warning

PFN flags are (unsigned long long), fix the alloc_dax_region() calling
convention to fix warnings of the form:

>> include/linux/pfn_t.h:18:17: warning: large integer implicitly truncated to unsigned type [-Woverflow]
    #define PFN_DEV (1ULL << (BITS_PER_LONG_LONG - 3))

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>