staging: lustre: mdc: document mdc_rpc_lock

staging: lustre: mdc: document mdc_rpc_lock

As this lock can be a bottleneck, clarifying why it is needed may be
helpful to those working on client performance.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3443
Reviewed-on: http://review.whamcloud.com/6593
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>