s390/mm,pageattr: fix walk_pte_level() early exit

In case of splitting to 4k mapping the early exit in walk_pte_level()
must only be taken iff flags is equal to SET_MEMORY_4K.
Currently the early exit is taken if the flag is set, and also others
might be set. This may lead to the situation that a mapping is split
but other changes are not done, like e.g. setting pages to R/W.

There is currently no such caller, but there might be in the future.

Fixes: b3e1a00c ("s390/mm: implement set_memory_4k()")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>