Resolve the devfs deadlock caused by LOR between devfs_mount->dm_lock and
vnode lock in devfs_allocv. Do this by temporary dropping dm_lock around
vnode locking.

For safe operation, add hold counters for both devfs_mount and devfs_dirent,
and DE_DOOMED flag for devfs_dirent. The facilities allow to continue after
dropping of the dm_lock, by making sure that referenced memory does not
disappear.

Reviewed by: tegge
Tested by: kris
Approved by: kan (mentor)
PR: kern/102335