rcu: only one evaluation of arg in rcu_dereference_check() unless sparse

The current version of the __rcu_access_pointer(), __rcu_dereference_check(),
and __rcu_dereference_protected() macros evaluate their "p" argument
three times, not counting typeof()s.  This is bad news if that argument
contains a side effect.  This commit therefore evaluates this argument
only once in normal kernel builds.  However, the straightforward approach
defeats sparse's RCU-pointer checking, so when __CHECKER__ is defined,
the additional pair of evaluations of the "p" argument are performed in
order to permit sparse to detect misuse of RCU-protected pointers.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>