GFS2: Truncate address space mapping when deleting an inode

In function gfs2_delete_inode() we write and flush the mapping for
a glock, among other things. We truncate the mapping for the inode,
but we never truncate the mapping for the glock. This patch makes it
also truncate the metamapping. This avoid cases where the glock is
reused by another process who is trying to recreate an inode in its
place using the same block.

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