nameicap_check_dotdot: trim tracker on check

Tracker should contain exactly the path from the starting directory to
the current lookup point. Otherwise we might not detect some cases of
dotdot escape. Consequently, if we are walking up the tree by dotdot
lookup, we must remove an entries below the walked directory.

Reviewed by: markj
Tested by: arichardson, pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D28907