des@ reported a panic in the NFSv4 server, where
nfsv4_fillattr() did a VOP_PATHCONF() without having
"vp" locked. kib@ suggested D51398 to fix it.
I think for the case of Readdirplus reply, relocking the
vnode can cause deadlocks. As such, this patch handles
VOP_PATHCONF() in the same way that the code checks
for ACL support, by doing the VOP_PATHCONF() before
the calls to nfsv4_filllattr() where the vnode is still locked.
des@, please test.