namei: stash the sampled ->d_seq into nameidata

New field: nd->next_seq.  Set to 0 outside of RCU mode, holds the sampled
value for the next dentry to be considered.  Used instead of an arseload
of local variables, arguments, etc.

step_into() has lost seq argument; nd->next_seq is used, so dentry passed
to it must be the one ->next_seq is about.

There are two requirements for RCU pathwalk:
	1) it should not give a hard failure (other than -ECHILD) unless
non-RCU pathwalk might fail that way given suitable timings.
	2) it should not succeed unless non-RCU pathwalk might succeed
with the same end location given suitable timings.

The use of seq numbers is the way we achieve that.  Invariant we want
to maintain is:
	if RCU pathwalk can reach the state with given nd->path, nd->inode
and nd->seq after having traversed some part of pathname, it must be possible
for non-RCU pathwalk to reach the same nd->path and nd->inode after having
traversed the same part of pathname, and observe the nd->path.dentry->d_seq
equal to what RCU pathwalk has in nd->seq

	For transition from parent to child, we sample child's ->d_seq
and verify that parent's ->d_seq remains unchanged.  Anything that
disrupts parent-child relationship would've bumped ->d_seq on both.
	For transitions from child to parent we sample parent's ->d_seq
and verify that child's ->d_seq has not changed.  Same reasoning as
for the previous case applies.
	For transition from mountpoint to root of mounted we sample
the ->d_seq of root and verify that nobody has touched mount_lock since
the beginning of pathwalk.  That guarantees that mount we'd found had
been there all along, with these mountpoint and root of the mounted.
It would be possible for a non-RCU pathwalk to reach the previous state,
find the same mount and observe its root at the moment we'd sampled
->d_seq of that
	For transitions from root of mounted to mountpoint we sample
->d_seq of mountpoint and verify that mount_lock had not been touched
since the beginning of pathwalk.  The same reasoning as in the
previous case applies.

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

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