This adds a new seq_...() helper:

   seq_path(seq_file, mnt, dentry, escape)

It spits the pathname into seq_file, does octal escapes for given set of
characters, returns the number of characters it'd produced or -1 in case
of error.  Long names are handled gracefully - you don't need anything
to do, generic seq_file logics will do the right thing.

/proc/mounts and /proc/swaps are converted to use of seq_path(), some
junk removed.

/proc/pid/maps will be converted next.