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>