GFS2: don't reference inode's glock during block allocation trace

This patch changes the block allocation trace so that it references
the rgd's glock rather than the inode's glock. Now that the order
of inode creation is switched, this prevents a reference to the
glock which may not be set yet.

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