gfs2: Fix an incorrect gfs2_assert()

When updating the inode information after a change in allocation,
convert the change into the same units as the inode's i_blocks count
before comparing it in an assertion.

Also, change the comparison so that it is still possible to set i_blocks
to zero by adding -i_blocks, something that was previously only possible
because of the difference in units.

Signed-off-by: Tim Smith <tim.smith@citrix.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>