xfs: xfs_buf_write_fail_rl_state can be static

Fix sparse warning introduced by commit ac8809f9

 ("xfs: abort
metadata writeback on permanent errors").

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>