[GFS2] Fix a size calculation error

Fix a size calculation error.
The size was incorrect being computed as a
negative length and then being passed to an
unsigned parameter.

This in turn would cause the allocator to
think it needed enough meta data to store
a gigabyte file for every file created.

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