GFS2: Don't try and dealloc own inode

When searching for unlinked, but still allocated inodes during block
allocation, avoid the block relating to the inode that is doing the
allocation. This fixes a hang caused when an unlinked, but still
open, inode tries to allocate some more blocks and lands up
finding itself during the search for deallocatable inodes.

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