ffs: do not return ESTALE on attempt to ffs_unotovp() on unlinked inode

The consequences are:

• for nfs exports and fhopen(2), unlinked but still referenced inodes are accessible
• for ffs_vput_pair() with unlock_vp = false, spurious ESTALE is not returned when the inode is still alive but unlinked

Note that tmpfs does not return ESTALE for the unlinked nodes.
The same behavior is claimed for Linux in
https://github.com/openzfs/zfs/issues/18699

Reviewed by: rmacklem
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D57982