gfs2: gfs2_inode_lookup cleanup

In gfs2_inode_lookup, once the inode has been looked up, we check if the
inode generation (no_formal_ino) is the one we're looking for.  If it
isn't and the inode wasn't in the inode cache, we discard the newly
looked up inode.  This is unnecessary, complicates the code, and makes
future changes to gfs2_inode_lookup harder, so change the code to retain
newly looked up inodes instead.

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