os: Plan 9: allocate space for a string in Rename

os: Plan 9: allocate space for a string in Rename

The Name field of the stat structure is variable length
and the marshalling code in package syscall requires
a buf long enough to contain the Name as well as the
static data. This change makes sure that the buffer in
os.Rename is allocated with the appropriate length.

R=rsc, rminnich, ality, r
CC=golang-dev
https://golang.org/cl/7453044