Fix a vnode locking bug in fuse_vnop_advlock.

Must lock the vnode before accessing the fufh table. Also, check for
invalid parameters earlier. Bug introduced by r346170.

MFC after: 2 weeks

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D27936

(cherry picked from commit 542711e52079f65647ac1daadf0c9e74cb221f3e)