x86/shadow: fix and improve sh_page_has_multiple_shadows()

While no caller currently invokes the function without first making sure
there is at least one shadow [1], we'd better eliminate UB here:
find_first_set_bit() requires input to be non-zero to return a well-
defined result.

Further, using find_first_set_bit() isn't very efficient in the first
place for the intended purpose.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

[1] The function has exactly two uses, and both are from OOS code, which
    is HVM-only. For HVM (but not for PV) sh_mfn_is_a_page_table(),
    guarding the call to sh_unsync(), guarantees at least one shadow.
    Hence even if sh_page_has_multiple_shadows() returned a bogus value
    when invoked for a PV domain, the subsequent is_hvm_vcpu() and
    oos_active checks (the former being redundant with the latter) will
    compensate. (Arguably that oos_active check should come first, for
    both clarity and efficiency reasons.)
master commit: 2896224a4e
master date: 2023-03-24 11:07:08 +0100

xen/arch/x86/mm/shadow/private.h

@@ -324,7 +324,7 @@ static inline int sh_page_has_multiple_shadows(struct page_info *pg)
         return 0;
     shadows = pg->shadow_flags & SHF_page_type_mask;
     /* More than one type bit set in shadow-flags? */
-    return ( (shadows & ~(1UL << find_first_set_bit(shadows))) != 0 );
+    return shadows && (shadows & (shadows - 1));
 }
 
 #if (SHADOW_OPTIMIZATIONS & SHOPT_OUT_OF_SYNC)