x86/mm: Micro-optimise clflush_cache_range()

Whilst inspecting the asm for clflush_cache_range() and some perf profiles
that required extensive flushing of single cachelines (from part of the
intel-gpu-tools GPU benchmarks), we noticed that gcc was reloading
boot_cpu_data.x86_clflush_size on every iteration of the loop. We can
manually hoist that read which perf regarded as taking ~25% of the
function time for a single cacheline flush.

Signed-off-by: Chris Wilson <[email protected]>
Reviewed-by: Ross Zwisler <[email protected]>
Acked-by: "H. Peter Anvin" <[email protected]>
Cc: Toshi Kani <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Luis R. Rodriguez <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Sai Praneeth <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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