GFS2: Add a bug trap in allocation code

This adds a check to ensure that if we reach the block allocator
that we don't try and proceed if there is no alloc structure
hanging off the inode. This should only happen if there is a bug
in GFS2. The error return code is distinctive in order that it
will be easily spotted.

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