s390/mm: make use of atomic_fetch_xor()

Make use of atomic_fetch_xor() instead of an atomic_cmpxchg() loop to
implement atomic_xor_bits() (aka atomic_xor_return()). This makes the C
code more readable and in addition generates better code, since for z196
and newer a single lax instruction is generated instead of a cmpxchg()
loop.
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/mm/pgalloc.c

@@ -133,13 +133,7 @@ int crst_table_upgrade(struct mm_struct *mm, unsigned long end)
 
 static inline unsigned int atomic_xor_bits(atomic_t *v, unsigned int bits)
 {
-	unsigned int old, new;
-
-	do {
-		old = atomic_read(v);
-		new = old ^ bits;
-	} while (atomic_cmpxchg(v, old, new) != old);
-	return new;
+	return atomic_fetch_xor(bits, v) ^ bits;
 }
 
 #ifdef CONFIG_PGSTE