Use whole mnt_stat.f_fsid bits for st_dev.

Since ino64 expanded dev_t to 64bit, make VOP_GETATTR(9) provide all
bits of mnt_stat.f_fsid as va_fsid for vnodes on filesystems which use
f_fsid. In particular, NFSv3 and sometimes NFSv4, and ZFS use this
method or reporting st_dev by stat(2).

Provide a new helper vn_fsid() to avoid duplicating code to copy
f_fsid to va_fsid.

Note that the change is mostly cosmetic. Its motivation is to avoid
sign-extension of f_fsid[0] into 64bit dev_t value which happens after
dev_t becomes 64bit..

Reviewed by: avg(zfs), rmacklem (nfs) (both for previous version)
Sponsored by: The FreeBSD Foundation