namei: ->d_inode of a pinned dentry is stable only for positives

namei: ->d_inode of a pinned dentry is stable only for positives

BugLink: http://bugs.launchpad.net/bugs/1553179
commit d4565649b6d6923369112758212b851adc407f0c upstream.

both do_last() and walk_component() risk picking a NULL inode out
of dentry about to become positive, *then* checking its flags and
seeing that it's not negative anymore and using (already stale by
then) value they'd fetched earlier.  Usually ends up oopsing soon
after that...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>