nfscommon: Use IS_DEFAULT_VNET() in the vnet initialization

Another oopsie. The vnet initialization function in
nfs_commonport.c for initializing prison0 by testing
curthread->td_ucred->cr_prison == &prison0. This is bogus
and always true. Replace it with IS_DEFAULT_VNET(curvnet).

(cherry picked from commit ef4e8f0cf91f7009745d5a7a90d3bdd2d9e25780)