Report symlink sizes from FUSE mount

Fixes #3667.

changelog/unreleased/issue-3667

@@ -0,0 +1,8 @@
+Bugfix: restic mount now reports symlinks sizes
+
+Symlinks used to have size zero in restic mountpoints, confusing some
+third-party tools. They now have a size equal to the byte length of their
+target path, as required by POSIX.
+
+https://github.com/restic/restic/issues/3667
+https://github.com/restic/restic/pull/3668

internal/fuse/link.go

@@ -1,3 +1,4 @@
+//go:build darwin || freebsd || linux
 // +build darwin freebsd linux
 
 package fuse
@@ -40,6 +41,8 @@ func (l *link) Attr(ctx context.Context, a *fuse.Attr) error {
 	a.Mtime = l.node.ModTime
 
 	a.Nlink = uint32(l.node.Links)
+	a.Size = uint64(len(l.node.LinkTarget))
+	a.Blocks = 1 + a.Size/blockSize
 
 	return nil
 }