fs: document seq_open()'s usage of file->private_data

seq_open() stores its struct seq_file in file->private_data, thus it must
not be modified by user of seq_file.

Link: http://lkml.kernel.org/r/cover.1433193673.git.ydroneaud@opteya.com


Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>