Add (and export) __cmpxchg_u16(), teach __cmpxchg() to use it.

And get rid of manual truncation down to u8, etc. in there - the
only reason for those is to avoid bogus warnings about constant
truncation from sparse, and those are easy to avoid by turning
that switch into conditional expression.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>