x86/bitops: adjust partial first word handling in __find_next{,_zero}_bit()

There's no need to subtract "bit" in what is passed to __scanbit(), as the
other bits are zero anyway after the shift (in __find_next_bit()) or can
be made so (in __find_next_zero_bit()) by flipping negation and shift. (We
actually leverage the same facts in find_next{,_zero}_bit() as well.) This
way in __scanbit() the TZCNT alternative can be engaged.

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

xen/arch/x86/bitops.c

@@ -35,8 +35,8 @@ unsigned int __find_next_bit(
     if ( bit != 0 )
     {
         /* Look for a bit in the first word. */
-        set = __scanbit(*p >> bit, BITS_PER_LONG - bit);
-        if ( set < (BITS_PER_LONG - bit) )
+        set = __scanbit(*p >> bit, BITS_PER_LONG);
+        if ( set < BITS_PER_LONG )
             return (offset + set);
         offset += BITS_PER_LONG - bit;
         p++;
@@ -85,8 +85,8 @@ unsigned int __find_next_zero_bit(
     if ( bit != 0 )
     {
         /* Look for zero in the first word. */
-        set = __scanbit(~(*p >> bit), BITS_PER_LONG - bit);
-        if ( set < (BITS_PER_LONG - bit) )
+        set = __scanbit(~*p >> bit, BITS_PER_LONG);
+        if ( set < BITS_PER_LONG )
             return (offset + set);
         offset += BITS_PER_LONG - bit;
         p++;