Factor out the code that creates an iovec from nfsvno_write().

This patch factors out the code that creates an iovec from nfsvno_write()
and modifies nfsvno_setxattr() to use it as well.
It also modifies nfsvno_readlink() to use the nfsrv_createiovec() function
already committed to create an iovec for nfsvno_read() and nfsvno_getxattr().