s390/mm: fix panic in gup_fast on large pud

On s390 there currently is no implementation of pud_write(). That was ok
as long as we had our own implementation of get_user_pages_fast() which
checked for pud protection by testing the bit directly w/o using
pud_write(). The other callers of pud_write() are not reachable on s390.

After commit 1a42010 ("s390/mm: convert to the generic
get_user_pages_fast code") we use the generic get_user_pages_fast(), which
does call pud_write() in pud_access_permitted() for FOLL_WRITE access on
a large pud. Without an s390 specific pud_write(), the generic version is
called, which contains a BUG() statement to remind us that we don't have a
proper implementation. This results in a kernel panic.

Fix this by providing an implementation of pud_write().

Cc: <stable@vger.kernel.org> # 5.2+
Fixes: 1a42010 ("s390/mm: convert to the generic get_user_pages_fast code")
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/pgtable.h

@@ -752,6 +752,12 @@ static inline int pmd_write(pmd_t pmd)
 	return (pmd_val(pmd) & _SEGMENT_ENTRY_WRITE) != 0;
 }
 
+#define pud_write pud_write
+static inline int pud_write(pud_t pud)
+{
+	return (pud_val(pud) & _REGION3_ENTRY_WRITE) != 0;
+}
+
 static inline int pmd_dirty(pmd_t pmd)
 {
 	return (pmd_val(pmd) & _SEGMENT_ENTRY_DIRTY) != 0;