ocfs2: Morph the haphazard OCFS2_IS_VALID_GROUP_DESC() checks.

Random places in the code would check a group descriptor bh to see if it
was valid. The previous commit unified descriptor block reads,
validating all block reads in the same place.  Thus, these checks are no
longer necessary.  Rather than eliminate them, however, we change them
to BUG_ON() checks.  This ensures the assumptions remain true.  All of
the code paths to these checks have been audited to ensure they come
from a validated descriptor read.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>