uprobes/x86: Cleanup the usage of arch_uprobe->def.fixups, make it u8

handle_riprel_insn() assumes that nobody else could modify ->fixups
before. This is correct but fragile, change it to use "|=".

Also make ->fixups u8, we are going to add the new members into the
union. It is not clear why UPROBE_FIX_RIP_.X lived in the upper byte,
redefine them so that they can fit into u8.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

arch/x86/include/asm/uprobes.h

@@ -53,7 +53,7 @@ struct arch_uprobe {
 #ifdef CONFIG_X86_64
 			long	riprel_target;
 #endif
-			u16	fixups;
+			u8	fixups;
 		} 			def;
 	};
 };