nfs_nfsdserv.c: Fix nfsrvd_openattr() for vnode locking

Without this patch, nfsrvd_openattr() requests an unlocked
vnode via VOP_LOOKUP(). This is not allowed for
"options DEBUG_VFS_LOCKS" kernels, so this patch requests a
locked vnode and then unlocks it.

Fixes: e4c7b2b6053f ("nfsv4: Add support to NFSv4 for named attributes")