Change nfs_advlock() so that the NFSVOPUNLOCK() is mostly done at the end.

Change nfs_advlock() so that the NFSVOPUNLOCK() is mostly done at the end.

Prior to this patch, nfs_advlock() did NFSVOPUNLOCK(); return (error);
in many places. This patch replaces these code sequenences with a "goto out;"
and does the NFSVOPUNLOCK(); return (error); at the end of the function
in order to make the vnode locking simpler.
This patch does not change the semantics of nfs_advlock().

Suggested by:	kib
Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D17853