nfsd4: fix error return when pseudoroot missing

We really shouldn't hit this case at all, and forthcoming kernel and
nfs-utils changes should eliminate this case; if it does happen,
consider it a bug rather than reporting an error that doesn't really
make sense for the operation (since there's no reason for a server to be
accepting v4 traffic yet have no root filehandle).

Also move some exp_pseudoroot code into a helper function while we're
here.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>