vfs: seq_file: add helpers for data filling

Add two helpers that allow access to the seq_file's own buffer, but
hide the internal details of seq_files.

This allows easier implementation of special purpose filling
functions.  It also cleans up some existing functions which duplicated
the seq_file logic.

Make these inline functions in seq_file.h, as suggested by Al.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>