unionfs: simplify writecount management

Use atomics to track the writecount granted to the underlying FS,
and avoid holding the vnode interlock while calling the underling FS'
VOP_ADD_WRITECOUNT(). This also fixes a WITNESS warning about nesting
the same lock type. Also add comments explaining why we need to track
the writecount on the unionfs vnode in the first place. Finally,
simplify writecount management to only use the upper vnode and assert
that we shouldn't have an active writecount on the lower vnode through
unionfs.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D33611