lift all calls of step_into() out of follow_dotdot/follow_dotdot_rcu

lift step_into() into handle_dots() (where they merge with each other);
have follow_... return dentry and pass inode/seq to the caller.

[braino fix folded; kudos to Qian Cai <cai@lca.pw> for reporting it]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>