make nameidata_dentry_drop_rcu_maybe() always leave RCU mode

Now we have do_follow_link() guaranteed to leave without dangling RCU
and the next step will get LOOKUP_RCU logics completely out of
link_path_walk().

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

diff --cc fs/namei.c
Simple merge