xfs: remove unnecessary null checks

The functions xfs_blkdev_put() and xfs_qm_dqrele() test whether
their argument is NULL and then return immediately.  Thus the test
around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>