[PATCH] get stack footprint of pathname resolution back to relative sanity

Somebody had put struct nameidata in stack frame of link_path_walk().
Unfortunately, there are certain realities to deal with:
	* It's in the middle of recursion.  Depth is equal to the nesting
depth of symlinks, i.e. up to 8.
	* struct namiedata is, even if one discards the intent junk,
at least 12 pointers + 5 ints.
	* moreover, adding a stack frame is not free in that situation.
	* there are fs methods called on top of that, and they also have
stack footprint.
	* kernel stack is not infinite.

The thing is, even if one chooses to deal with -ESTALE that way (and it's
one hell of an overkill), the only thing that needs to be preserved is
vfsmount + dentry, not the entire struct nameidata.

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