locking/atomics: Update comment about READ_ONCE() and structures

The comment is out of data. Also point out the performance drawback
of the barrier();__builtin_memcpy(); barrier() followed by another
copy from stack (__u) to lvalue;

Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
[ Made it a bit more readable. ]
Signed-off-by: Ingo Molnar <[email protected]>

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