gfs2: Fix mmap locking for write faults

When a write fault occurs, we need to take the inode glock of the underlying
inode in exclusive mode.  Otherwise, there's no guarantee that the dirty page
will be written back to disk.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/file.c

@@ -540,9 +540,11 @@ static vm_fault_t gfs2_fault(struct vm_fault *vmf)
 	struct gfs2_inode *ip = GFS2_I(inode);
 	struct gfs2_holder gh;
 	vm_fault_t ret;
+	u16 state;
 	int err;
 
-	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
+	state = (vmf->flags & FAULT_FLAG_WRITE) ? LM_ST_EXCLUSIVE : LM_ST_SHARED;
+	gfs2_holder_init(ip->i_gl, state, 0, &gh);
 	err = gfs2_glock_nq(&gh);
 	if (err) {
 		ret = block_page_mkwrite_return(err);