x86/gnttab: Use explicit instruction size in gnttab_clear_flags()

x86/gnttab: Use explicit instruction size in gnttab_clear_flags()

The OpenSUSE Leap compilers complain about ambiguity:

In file included from grant_table.c:33:
In file included from ...xen/include/xen/grant_table.h:30:
...xen/include/asm/grant_table.h:67:19: error: ambiguous instructions require
an explicit suffix (could be 'andb', 'andw', 'andl', or 'andq')
    asm volatile ("lock and %1,%0" : "+m" (*addr) : "ir" ((uint16_t)~mask));
                  ^
<inline asm>:1:2: note: instantiated into assembly here
        lock and $-17,(%rsi)
        ^

Full logs: https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/247600284
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>