vfs: __d_path: dont prepend the name of the root dentry

vfs: __d_path: dont prepend the name of the root dentry

In the old times pseudo-filesystems set the name of theroot dentry to
some prefix like "pipe:" and the name of the child dentry to "[123]"
and relied on a hack in __d_path() to replace the preceding slash with
the root's name to get "pipe:[123]".

Then the d_dname() dentry operation was introduced which solved the
same problem without having to pre-fill the name in each dentry.

Currently the following pseudo filesystems exist in the kernel:

perfmon
mtd
anon_inode
bdev
pipe
socket

Of these only perfmon, anon_inode, pipe and socket create
sub-dentries, all of which have now been switched to using d_dname().

bdev and mtd only create inodes.

This means that now the hack to overwrite the slash can be removed, so
for unreachable paths (e.g. within a detached mount) the path string
won't be polluted with garbage.  For these cases a subsequent patch
will add a prefix, indicating that the path is unreachable.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>