dax: fallback from pmd to pte on error

In preparation for consulting a badblocks list in pmem_direct_access(),
teach dax_pmd_fault() to fallback rather than fail immediately upon
encountering an error.  The thought being that reducing the span of the
dax request may avoid the error region.
Reviewed-by: NJeff Moyer <jmoyer@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJan Kara <jack@suse.cz>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NVishal Verma <vishal.l.verma@intel.com>

fs/dax.c

@@ -855,8 +855,8 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 		long length = dax_map_atomic(bdev, &dax);
 
 		if (length < 0) {
-			result = VM_FAULT_SIGBUS;
-			goto out;
+			dax_pmd_dbg(&bh, address, "dax-error fallback");
+			goto fallback;
 		}
 		if (length < PMD_SIZE) {
 			dax_pmd_dbg(&bh, address, "dax-length too small");