srcu: Remove extraneous parentheses from srcu_read_lock() etc.

This commit removes extraneous parentheses from srcu_read_lock(),
srcu_read_lock_nmisafe(), srcu_read_unlock(), and
srcu_read_unlock_nmisafe().  Looks like someone was once a macro.

Cc: Christoph Hellwig <hch@lst.de>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: "Zhang, Qiang1" <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>