Documentation: Add "@" in front of private structure members.

Even "private" structure members need a leading "@" in their
kernel-doc; otherwise, they will be treated as new section names in
the resulting manual.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>