fs: properly handle ENOTSUP in copyXAttrs

[sondavidb]

Fixes #244. More context in the issue.

Filesystems without xattr support do not have any xattrs to copy. The syscall for this will return ENOTSUP to indicate this, but continuity will treat it as a regular error. This change will return nil if this call returns ENOTSUP.

[estesp]

haven't had time to look deeper, but this CI test failure is strange; I don't think we have flakes in this project, so is this being caused by the change:

=== RUN   TestDiffDirChangeWithOverlayfs
    diff_test.go:249: failed diff dir change: Unexpected number of changes:
        got(5):
        	modify	/dir1
        	delete	/dir1/d
        	modify	/dir1/f
        	modify	/dir2/d/f
        	delete	/dir3/.wh..opq
        expected(8):
        	modify	/dir1
        	delete	/dir1/d
        	modify	/dir1/f
        	modify	/dir2
        	modify	/dir2/d
        	modify	/dir2/d/f
        	modify	/dir3
        	delete	/dir3/.wh..opq
--- FAIL: TestDiffDirChangeWithOverlayfs (0.00s)

[thaJeztah]

@estesp I think I saw the same failure on https://github.com/containerd/continuity/pull/242

wondering if it's changes on the runners

[AkihiroSuda]

Please rebase to pass the CI