srcu: Move ->srcu_gp_mutex from srcu_struct to srcu_usage

This commit moves the ->srcu_gp_mutex field from the srcu_struct structure
to the srcu_usage structure to reduce the size of the former in order
to improve cache locality.

Suggested-by: Christoph Hellwig <hch@lst.de>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: "Zhang, Qiang1" <qiang1.zhang@intel.com>
Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>