gfs2: Clean up inode initialization and teardown

When allocating a new inode, mark the iopen glock holder as uninitialized to
make sure gfs2_evict_inode won't fail after an incomplete create or lookup.  In
gfs2_evict_inode, allow the inode glock to be NULL and remove the duplicate
iopen glock teardown code.  In gfs2_inode_lookup, don't tear down things that
gfs2_evict_inode will already tear down.

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