remove unused bh in calls to ext234_get_group_desc

ext[234]_get_group_desc never tests the bh argument, and only sets it if it
is passed in; it is perfectly happy with a NULL bh argument.  But, many
callers send one in and never use it.  May as well call with NULL like
other callers who don't use the bh.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>