x86/segment: Use MOVL when reading segment registers

Use MOVL when reading segment registers to avoid 0x66 operand-size override
insn prefix. The segment value is always 16-bit and gets zero-extended to the
full 32-bit size.

Example:

  4e4:       66 8c c0                mov    %es,%ax
  4e7:       66 89 83 80 0b 00 00    mov    %ax,0xb80(%rbx)

  4e4:       8c c0                   mov    %es,%eax
  4e6:       66 89 83 80 0b 00 00    mov    %ax,0xb80(%rbx)

Also, use the %k0 modifier which generates the SImode (signed integer)
register name for the target register.

  [ bp: Extend and clarify commit message. ]

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Link: https://patch.msgid.link/20260105090422.6243-1-ubizjak@gmail.com

Author: ubizjak

arch/x86/include/asm/segment.h

@@ -348,7 +348,7 @@ static inline void __loadsegment_fs(unsigned short value)
  * Save a segment register away:
  */
 #define savesegment(seg, value)				\
-	asm("mov %%" #seg ",%0":"=r" (value) : : "memory")
+	asm("movl %%" #seg ",%k0" : "=r" (value) : : "memory")
 
 #endif /* !__ASSEMBLER__ */
 #endif /* __KERNEL__ */