checkpatch: improve parse_email signature checking

Bare email addresses with non alphanumeric characters require escape
quoting before being substituted in the parse_email routine.

e.g. Reported-by:  <syzbot+bbd8e9a06452cc48059b@syzkaller.appspotmail.com>

Do so.

Link: http://lkml.kernel.org/r/1518631805.3678.12.camel@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>