zfs: Fix a pair of bugs in zfs_fhtovp()

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.

Approved by:	so
PR:		266236
Security:	FreeBSD-EN-22:24.zfs

(cherry picked from commit 6fe0a6c80a1aff14236924eb33e4013aa8c14f91)
(cherry picked from commit 562c9ac58c7678b13f52b0bfe63148e40d7bf63d)