v2api: properly implement Node.Setattr

We used to always open a *File2 and letting the *File2
handle Setattr. This does not work it we cannot open the file!

Before:

    $ go test
    2020/07/12 20:14:57 writer: Write/Writev failed, err: 2=no such file or directory. opcode: INTERRUPT
    2020/07/12 20:14:57 writer: Write/Writev failed, err: 2=no such file or directory. opcode: INTERRUPT
    --- FAIL: TestLchown (0.00s)
        matrix_test.go:634: lchown /tmp/gocryptfs-test-parent-1026/014500839/default-plain/symlink: too many levels of symbolic links
    touch: setting times of '/tmp/gocryptfs-test-parent-1026/014500839/default-plain/utimesnano_symlink': Too many levels of symbolic links
    --- FAIL: TestUtimesNanoSymlink (0.00s)
        matrix_test.go:655: exit status 1
    --- FAIL: TestMkfifo (0.00s)
        matrix_test.go:755: file exists
    --- FAIL: TestMagicNames (0.00s)
        matrix_test.go:773: Testing n="gocryptfs.longname.QhUr5d9FHerwEs--muUs6_80cy6JRp89c1otLwp92Cs"
        matrix_test.go:773: Testing n="gocryptfs.diriv"
        matrix_test.go:815: open /tmp/gocryptfs-test-parent-1026/014500839/default-plain/linktarget: permission denied
    --- FAIL: TestChmod (0.00s)
        matrix_test.go:840: chmod 444 -> 000 failed: permission denied
        matrix_test.go:840: chmod 444 -> 111 failed: permission denied
        matrix_test.go:840: chmod 444 -> 123 failed: permission denied
        matrix_test.go:840: chmod 444 -> 321 failed: permission denied
    FAIL
    exit status 1
    FAIL	github.com/rfjakob/gocryptfs/tests/matrix	0.790s

After:

    $ go test
    --- FAIL: TestMkfifo (0.00s)
        matrix_test.go:755: file exists
    --- FAIL: TestMagicNames (0.00s)
        matrix_test.go:773: Testing n="gocryptfs.longname.QhUr5d9FHerwEs--muUs6_80cy6JRp89c1otLwp92Cs"
        matrix_test.go:773: Testing n="gocryptfs.diriv"
        matrix_test.go:815: open /tmp/gocryptfs-test-parent-1026/501766059/default-plain/linktarget: permission denied
    --- FAIL: TestChmod (0.00s)
        matrix_test.go:849: modeHave 0644 != modeWant 0
    FAIL
    exit status 1
    FAIL    github.com/rfjakob/gocryptfs/tests/matrix   0.787s

0 files changed, 0 insertions, 0 deletions