vfs: reimplement deferred inactive to use a dedicated flag (VI_DEFINACT)

vfs: reimplement deferred inactive to use a dedicated flag (VI_DEFINACT)

The previous behavior of leaving VI_OWEINACT vnodes on the active list without
a hold count is eliminated. Hold count is kept and inactive processing gets
explicitly deferred by setting the VI_DEFINACT flag. The syncer is then
responsible for vdrop.

Reviewed by:	kib (previous version)
Tested by:	pho (in a larger patch, previous version)
Differential Revision:	https://reviews.freebsd.org/D23036