gfs2: Move inode generation number check into gfs2_inode_lookup

Move the inode generation number check from gfs2_lookup_by_inum into
gfs2_inode_lookup: gfs2_inode_lookup may be able to decide that an inode with
the given inode generation number cannot exist without having to verify the
block type or reading the inode from disk.

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