gfs2: Large-filesystem fix for 32-bit systems

Commit ff34245d switched from iget5_locked to iget_locked among other
things, but iget_locked doesn't work for filesystems larger than 2^32
blocks on 32-bit systems.  Switch back to iget5_locked.  Filesystems
larger than 2^32 blocks are unrealistic to work well on 32-bit systems,
so this is mostly a code cleanliness fix.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>