dax: Fix stack overflow when mounting fsdax pmem device

dax: Fix stack overflow when mounting fsdax pmem device

When mounting fsdax pmem device, commit 6180bb446ab6 ("dax: fix
detection of dax support for non-persistent memory block devices")
introduces the stack overflow [1][2]. Here is the call path for
mounting ext4 file system:
  ext4_fill_super
    bdev_dax_supported
      __bdev_dax_supported
        dax_supported
          generic_fsdax_supported
            __generic_fsdax_supported
              bdev_dax_supported

The call path leads to the infinite calling loop, so we cannot
call bdev_dax_supported() in __generic_fsdax_supported(). The sanity
checking of the variable 'dax_dev' is moved prior to the two
bdev_dax_pgoff() checks [3][4].

[1] https://lore.kernel.org/linux-nvdimm/1420999447.1004543.1600055488770.JavaMail.zimbra@redhat.com/
[2] https://lore.kernel.org/linux-nvdimm/alpine.LRH.2.02.2009141131220.30651@file01.intranet.prod.int.rdu2.redhat.com/
[3] https://lore.kernel.org/linux-nvdimm/CA+RJvhxBHriCuJhm-D8NvJRe3h2MLM+ZMFgjeJjrRPerMRLvdg@mail.gmail.com/
[4] https://lore.kernel.org/linux-nvdimm/20200903160608.GU878166@iweiny-DESK2.sc.intel.com/

Fixes: 6180bb446ab6 ("dax: fix detection of dax support for non-persistent memory block devices")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Ritesh Harjani <riteshh@linux.ibm.com>
Cc: Coly Li <colyli@suse.de>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: John Pittman <jpittman@redhat.com>
Link: https://lore.kernel.org/r/20200917111549.6367-1-adrianhuang0701@gmail.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>