get_maintainer: quote email address with period

Picky mail systems won't accept email addresses where recipient has period
in name; ie.  David S.  Miller <davemloft.net> will not work.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>