util(disk): freebsd and netbsd support

[crazy-max]

Was looking at netbsd support in buildx and it fails to build the util/disk package:

#18 [buildx-build 1/1] RUN --mount=type=bind,target=.   --mount=type=cache,target=/root/.cache   --mount=type=cache,target=/go/pkg/mod   --mount=type=bind,from=buildx-version,source=/buildx-version,target=/buildx-version <<EOT (set -e...)
#18 0.322 + CGO_ENABLED=0 go build -mod vendor -trimpath -ldflags '-X github.com/docker/buildx/version.Version=v0.19.0-49-g3e1a0982 -X github.com/docker/buildx/version.Revision=3e1a0982b6216e234e5576a5a30c30bf05ed4a6e -X github.com/docker/buildx/version.Package=github.com/docker/buildx -s -w' -o /usr/bin/docker-buildx ./cmd/buildx
#18 1.142 # github.com/moby/buildkit/util/disk
#18 1.142 vendor/github.com/moby/buildkit/util/disk/disk_unix.go:13:20: undefined: syscall.Statfs
#18 1.142 vendor/github.com/moby/buildkit/util/disk/disk_unix.go:18:23: st.Bsize undefined (type syscall.Statfs_t has no field or method Bsize)
#18 1.142 vendor/github.com/moby/buildkit/util/disk/disk_unix.go:18:41: st.Blocks undefined (type syscall.Statfs_t has no field or method Blocks)
#18 1.142 vendor/github.com/moby/buildkit/util/disk/disk_unix.go:19:23: st.Bsize undefined (type syscall.Statfs_t has no field or method Bsize)
#18 1.142 vendor/github.com/moby/buildkit/util/disk/disk_unix.go:19:41: st.Bfree undefined (type syscall.Statfs_t has no field or method Bfree)
#18 1.142 vendor/github.com/moby/buildkit/util/disk/disk_unix.go:20:23: st.Bsize undefined (type syscall.Statfs_t has no field or method Bsize)
#18 1.142 vendor/github.com/moby/buildkit/util/disk/disk_unix.go:20:41: st.Bavail undefined (type syscall.Statfs_t has no field or method Bavail)

cc @akhramov @catap