xfs: fix SB_BORN check in xfs_dax_notify_failure()

The SB_BORN flag is stored in the vfs superblock, not xfs_sb.

Link: https://lkml.kernel.org/r/166153428094.2758201.7936572520826540019.stgit@dwillia2-xfh.jf.intel.com
Fixes: 6f643c57 ("xfs: implement ->notify_failure() for XFS")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Goldwyn Rodrigues <rgoldwyn@suse.de>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/xfs/xfs_notify_failure.c

@@ -175,7 +175,7 @@ xfs_dax_notify_failure(
 	u64			ddev_start;
 	u64			ddev_end;
 
-	if (!(mp->m_sb.sb_flags & SB_BORN)) {
+	if (!(mp->m_super->s_flags & SB_BORN)) {
 		xfs_warn(mp, "filesystem is not ready for notify_failure()!");
 		return -EIO;
 	}