Introduce __read_frequently

While __read_mostly groups variables together, their placement is not
specified. In particular 2 frequently used variables can end up in
different lines.

This annotation is only expected to be used for variables read all the time,
e.g. on each syscall entry.

Sprinkle __read_frequently on few obvious places.

Note that some of annotated variables should probably change their types
to something smaller, preferably bit-sized.

Mark kdb_active as __read_frequently and switch to bool to eat less space.

Change kdb_active type to u_char.

Fixes warnings from gcc and keeps the small size. Perhaps nesting should be moved
to another variablle.