[GFS2] gfs2_dir_read_data(): fix uninitialized variable usage

In the "if (extlen)" case, "bh" was used uninitialized.

This patch changes the code to what seems to have been intended.

Spotted by the Coverity checker.

This patch also removes a pointless "bh = NULL" asignment (the variable
is never accessed again after this point).

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>