seq_file: reuse string_escape_str()

strint_escape_str() escapes input string by given criteria.  In case of
seq_escape() the criteria is to convert some characters to their octal
representation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>