kernel-doc: escape xml for structs

scripts/kernel-doc was leaving unescaped '<', '>', and '&' in
generated xml output for structs.  This causes xml parser errors.
Convert these characters to "&lt;", "&gt;", and "&amp;" as needed
to prevent errors.

Most of the conversion was already done; complete it just before
output.

Documentation/DocBook/device-drivers.xml:41883: parser error : StartTag: invalid element name
#define INPUT_KEYMAP_BY_INDEX	(1 << 0)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>