fs/xfs: Create function xfs_inode_should_enable_dax()
xfs_inode_supports_dax() should reflect if the inode can support DAX not
that it is enabled for DAX.
Change the use of xfs_inode_supports_dax() to reflect only if the inode
and underlying storage support dax.
Add a new function xfs_inode_should_enable_dax() which reflects if the
inode should be enabled for DAX.
Reviewed-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Ira Weiny <ira.weiny@intel.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>