btrfs: depend on kernel UAPI instead of libbtrfs

See containerd/btrfs PR 40 and moby/moby PR 44761. (Thanks to [@]neersighted.)

The containerd/btrfs library now requires headers from kernel 4.12 or newer:
- https://github.com/torvalds/linux/blob/master/include/uapi/linux/btrfs.h
- https://github.com/torvalds/linux/blob/master/include/uapi/linux/btrfs_tree.h

These files are licensed under the GPL-2.0 WITH Linux-syscall-note, so it should be compatible with the Apache License 2.0.
https://spdx.org/licenses/Linux-syscall-note.html

The dependency on the kernel headers only affects users building from source.
Users on older kernels may opt to not compile this library (`BUILDTAGS=no_btfs`),
or to provide headers from a newer kernel.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

vendor/golang.org/x/sys/unix/syscall_linux.go

@@ -1800,6 +1800,7 @@ func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 //sysnb	Capset(hdr *CapUserHeader, data *CapUserData) (err error)
 //sys	Chdir(path string) (err error)
 //sys	Chroot(path string) (err error)
+//sys	ClockAdjtime(clockid int32, buf *Timex) (state int, err error)
 //sys	ClockGetres(clockid int32, res *Timespec) (err error)
 //sys	ClockGettime(clockid int32, time *Timespec) (err error)
 //sys	ClockNanosleep(clockid int32, flags int, request *Timespec, remain *Timespec) (err error)
@@ -1999,7 +2000,7 @@ func appendBytes(vecs []Iovec, bs [][]byte) []Iovec {
 // offs2lohi splits offs into its low and high order bits.
 func offs2lohi(offs int64) (lo, hi uintptr) {
 	const longBits = SizeofLong * 8
-	return uintptr(offs), uintptr(uint64(offs) >> longBits)
+	return uintptr(offs), uintptr(uint64(offs) >> (longBits - 1) >> 1) // two shifts to avoid false positive in vet
 }
 
 func Readv(fd int, iovs [][]byte) (n int, err error) {