pkg/system: handle changed size case

lgetxattr(2) man page says:

> If size is specified as zero, these calls return the  current  size  of
> the  named extended attribute (and leave value unchanged).  This can be
> used to determine the size of the buffer that should be supplied  in  a
> subsequent  call.   (But, bear in mind that there is a possibility that
> the attribute value may change between the two calls,  so  that  it  is
> still necessary to check the return status from the second call.)

The current code does not handle the "bear in mind" case, i.e. when
the size changes between the two calls, and the new size is larger.

Fix the above problem, and slightly simplify the code.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

pkg/system/xattrs_linux.go

@@ -16,30 +16,29 @@ const (
 
 // Lgetxattr retrieves the value of the extended attribute identified by attr
 // and associated with the given path in the file system.
-// It will returns a nil slice and nil error if the xattr is not set.
+// Returns a []byte slice if the xattr is set and nil otherwise.
 func Lgetxattr(path string, attr string) ([]byte, error) {
 	// Start with a 128 length byte array
 	dest := make([]byte, 128)
 	sz, errno := unix.Lgetxattr(path, attr, dest)
 
-	switch {
-	case errno == unix.ENODATA:
-		return nil, nil
-	case errno == unix.ERANGE:
-		// 128 byte array might just not be good enough. A dummy buffer is used
-		// to get the real size of the xattrs on disk
+	for errno == unix.ERANGE {
+		// Buffer too small, use zero-sized buffer to get the actual size
 		sz, errno = unix.Lgetxattr(path, attr, []byte{})
 		if errno != nil {
 			return nil, errno
 		}
 		dest = make([]byte, sz)
 		sz, errno = unix.Lgetxattr(path, attr, dest)
-		if errno != nil {
-			return nil, errno
-		}
+	}
+
+	switch {
+	case errno == unix.ENODATA:
+		return nil, nil
 	case errno != nil:
 		return nil, errno
 	}
+
 	return dest[:sz], nil
 }