VFS: remove MNTK_MARKER

We no longer allow upper filesystems to be unregistered from the base
mount while vfs_notify_upper() or any other upper operation is pending.
New upper mounts can still be registered during this period, but they
will be added at the end of the upper mount tailq. We therefore no
longer need to allocate marker nodes during vfs_notify_upper() to keep
our place in the iteration.

Reviewed by: kib, mckusick
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D31016