[FreeBSD] zfs_znode_alloc: lock the vnode earlier

This is needed because of a possible error path where zfs_vnode_forget()
is called. That function calls vgone() and vput(), the former requires
the vnode to be exclusively locked and the latter expects it to be
locked.

It should be safe to lock the vnode as early as possible because it is
not yet visible, so there is no interaction with other locks.

While here, remove a tautological assignment to 'vp'.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Andriy Gapon <avg@FreeBSD.org>
Closes #14565