dax: fix offset overflow in dax_io

This isn't functionally apparent for some reason, but
when we test io at extreme offsets at the end of the loff_t
rang, such as in fstests xfs/071, the calculation of
"max" in dax_io() can be wrong due to pos + size overflowing.

For example,

# xfs_io -c "pwrite 9223372036854771712 512" /mnt/test/file

enters dax_io with:

start 0x7ffffffffffff000
end   0x7ffffffffffff200

and the rounded up "size" variable is 0x1000.  This yields:

pos + size 0x8000000000000000 (overflows loff_t)
       end 0x7ffffffffffff200

Due to the overflow, the min() function picks the wrong
value for the "max" variable, and when we send (max - pos)
into i.e. copy_from_iter_pmem() it is also the wrong value.

This somehow(tm) gets magically absorbed without incident,
probably because iter->count is correct.  But it seems best
to fix it up properly by comparing the two values as
unsigned.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>