x86/percpu: Convert this_percpu_xchg_op() from asm() to C code, to generate better...

x86/percpu: Convert this_percpu_xchg_op() from asm() to C code, to generate better code

Rewrite percpu_xchg_op() using generic percpu primitives instead
of using asm. The new implementation is similar to local_xchg() and
allows the compiler to perform various optimizations: e.g. the
compiler is able to create fast path through the loop, according
to likely/unlikely annotations in percpu_try_cmpxchg_op().

No functional changes intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240320083127.493250-1-ubizjak@gmail.com