nfsclient: Avoid copying uninitialized bytes into statfs

[markj]

Summary

hst will be nul-terminated but may be followed by uninitialized bytes.
Avoid copying all of them since they are exported to userland via
statfs(2).

Reported by: KMSAN
MFC after: 2 weeks

[rmacklem]

This looks ok to me. It does assume that "mnt_stat" is initialized '0', but that is the case.

Alternately, you could add M_ZERO to the malloc() for hst, and then it doesn't matter
if mnt_stat is properly initialized.

I'll let you decide which you prefer.

[markj]

In D31167#701453, @rmacklem wrote:

This looks ok to me. It does assume that "mnt_stat" is initialized '0', but that is the case.

Alternately, you could add M_ZERO to the malloc() for hst, and then it doesn't matter
if mnt_stat is properly initialized.

I'll let you decide which you prefer.

I thought about using M_ZERO but I think this approach is more internally consistent: there are other strings allocated when mounting that don't require M_ZERO, and NFS itself doesn't care whether the hostname buffer is zeroed.