s390/pgtable: Use set_pmd_bit() to invalidate PMD entry

s390/pgtable: Use set_pmd_bit() to invalidate PMD entry

Commit 3a5a8d343e1c ("mm: fix race between __split_huge_pmd_locked()
and GUP-fast") failed to follow the convention and used direct PMD
entry modification instead of set_pmd_bit().

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Link: https://lore.kernel.org/r/a9248694a38cc898d3f0628f59b8abb57d56a416.1772812343.git.agordeev@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>