gfs2: Delete an unnecessary check before brelse()

The brelse() function tests whether its argument is NULL and then
returns immediately.  Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

[The same applies to brelse() in gfs2_dir_no_add (which Coccinelle
apparently missed), so fix that as well.]

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>