[mszeredi@suse.cz] split big patch into managable chunks

Add the following functions:

  dentry_path()
  seq_dentry()

These are similar to d_path() and seq_path().  But instead of
calculating the path within a mount namespace, they calculate the path
from the root of the filesystem to a given dentry, ignoring mounts
completely.
Signed-off-by: NRam Pai <linuxram@us.ibm.com>
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>