um: disable CMPXCHG_DOUBLE as it breaks UML build

Commit b789ef518b2 ("slub: Add cmpxchg_double_slab()") tests for
cmpxchg_double support in the SLUB code and it breaks UML builds with
SLUB. Since UML does not support checking for CPU features, disable
CMPXCHG_DOUBLE just like CMPXCHG_LOCAL is disabled for UML.

Signed-off-by: Thadeu Lima de Souza Cascardo <[e-mail]>
Reviewed-by: Christoph Lameter <[e-mail]>
Cc: Jeff Dike <[e-mail]>
Cc: Richard Weinberger <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/um/Kconfig.x86

···
 	bool
 	default n
 
+config CMPXCHG_DOUBLE
+	bool
+	default n
+
 source "arch/x86/Kconfig.cpu"
 
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