gfs2: Add missing else in trans_add_meta/data

This patch fixes a timing window that causes a segfault.
The problem is that bd can remain NULL throughout the function
and then reference that NULL pointer if the bh->b_private starts
out NULL, then someone sets it to non-NULL inside the locking.
In that case, bd still needs to be set.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>