Fix "vrefact: wrong use count 0" with DRM

Bump the vnode use count, not its hold count. This fixes a panic triggered
by fstatat(..., AT_EMPTY_PATH) on DRM device nodes, which happens to be
what glxinfo(1) from Ubuntu Jammy is doing.

PR: kern/274538
Reviewed By: kib (earlier version), olce
Differential Revision: https://reviews.freebsd.org/D47391