x86/asm: Use asm_inline() instead of asm() in clwb()

Use asm_inline() to instruct the compiler that the size of asm()
is the minimum size of one instruction, ignoring how many instructions
the compiler thinks it is. ALTERNATIVE macro that expands to several
pseudo directives causes instruction length estimate to count
more than 20 instructions.

bloat-o-meter reports slight increase of the code size
for x86_64 defconfig object file, compiled with gcc-14.2:

add/remove: 0/2 grow/shrink: 3/0 up/down: 190/-59 (131)

Function old new delta
__copy_user_flushcache 166 247 +81
__memcpy_flushcache 369 437 +68
arch_wb_cache_pmem 6 47 +41
__pfx_clean_cache_range 16 - -16
clean_cache_range 43 - -43

Total: Before=22807167, After=22807298, chg +0.00%

The compiler now inlines and removes the clean_cache_range() function.

Signed-off-by: Uros Bizjak <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/special_insns.h

···
 {
 	volatile struct { char x[64]; } *p = __p;
 
-	asm volatile(ALTERNATIVE_2(
+	asm_inline volatile(ALTERNATIVE_2(
 		"ds clflush %0",
 		"clflushopt %0", X86_FEATURE_CLFLUSHOPT,
 		"clwb %0", X86_FEATURE_CLWB)