locking/Documentation: Update locking/mutex-design.txt disadvantages

Fortunately Jason was able to reduce some of the overhead we
had introduced in the original rwsem optimistic spinning -
an it is now the same size as mutexes. Update the documentation
accordingly.
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Acked-by: Jason Low <jason.low2@hp.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: aswin@hp.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: http://lkml.kernel.org/r/1406752916-3341-7-git-send-email-davidlohr@hp.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/locking/mutex-design.txt

@@ -145,9 +145,9 @@ Disadvantages
 
 Unlike its original design and purpose, 'struct mutex' is larger than
 most locks in the kernel. E.g: on x86-64 it is 40 bytes, almost twice
-as large as 'struct semaphore' (24 bytes) and 8 bytes shy of the
-'struct rw_semaphore' variant. Larger structure sizes mean more CPU
-cache and memory footprint.
+as large as 'struct semaphore' (24 bytes) and tied, along with rwsems,
+for the largest lock in the kernel. Larger structure sizes mean more
+CPU cache and memory footprint.
 
 When to use mutexes
 -------------------