nfsclient: Avoid copying uninitialized bytes into statfs

hst will be nul-terminated but the remaining space in the buffer is left
uninitialized. Avoid copying the entire buffer to ensure that
uninitialized bytes are not leaked via statfs(2).

Reported by: KMSAN
Reviewed by: rmacklem
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31167