LU-5946 lprocfs: cleanup stats locking code

LU-5946 lprocfs: cleanup stats locking code

Add comment blocks on lprocfs_stats_lock() and lprocfs_stats_unlock().
Move common NOPERCPU code out of the switch() statements to reduce
code size and complexity, since it doesn't depend on the opc at all.

Replace switch() in lprocfs_stats_unlock() with a simple if/else,
since the lock opc was already checked in lprocfs_stats_lock().

Add an enum for the lprocfs_stats_lock() operations to make it clear
what the valid values are and allow compiler checking.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I1906d44981c025964d6a510000b217f10ca23031
Reviewed-on: http://review.whamcloud.com/12872
Tested-by: Jenkins
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>