zfs: Fix a pair of bugs in zfs_fhtovp()

This cherry-picks upstream ed566bf1cd0bdbf85e8c63c1c119e3d2ef5db1f6

• Add a zfs_exit() call in an error path, otherwise a lock is leaked.
• Remove the fid_gen > 1 check. That appears to be Linux-specific: zfsctl_snapdir_fid() sets fid_gen to 0 or 1 depending on whether the snapshot directory is mounted. On FreeBSD it fails, making snapshot dirs inaccessible via NFS.

PR: 266236
MFC after: 3 days