scm: use masked_user_access_begin() in put_cmsg()

Use the greatest and latest uaccess construct to get an optimal code.

Before :

	lea    (%r9,%rcx,1),%r10
	movabs $<USER_PTR_MAX>,%r11
	mov    $0xfffffff2,%eax
	cmp    %rcx,%r10
	jb     ffffffff81cdc312 <put_cmsg+0x152>
	cmp    %r11,%r10
	ja     ffffffff81cdc312 <put_cmsg+0x152>
	stac
	lfence
	mov    %r9,(%rcx)

After:

	movabs $<USER_PTR_MAX>,%r9
	cmp    %r9,%rax
	cmova  %r9,%rax
	stac
	mov    %rcx,(%rax)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250925224914.3590290-1-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>