nfs_clvnops.c: Add an initialization mistakenly removed

Commit e4c7b2b6053f mistakenly removed an initialization of "newvp",
which could result in crashes.

This patch puts the initialization back in.

Reported by: cy
Tested by: cy
Fixes: e4c7b2b6053f ("nfsv4: Add support to NFSv4 for named attributes")