nilfs2: remove constant qualifier from argument of bmap propagate

The first argument of bops->bop_propagate operation takes a constant
qualifier, and causes compilation error when removed cast to pointer
of nilfs_btree structure type.  This fixes the issue to prepare for
succesive removal of nilfs_btree struct.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/direct.c b/fs/nilfs2/direct.c
index 32f1746..fd006ee 100644
--- a/fs/nilfs2/direct.c
+++ b/fs/nilfs2/direct.c
@@ -269,7 +269,7 @@
 	return 0;
 }
 
-static int nilfs_direct_propagate(const struct nilfs_bmap *bmap,
+static int nilfs_direct_propagate(struct nilfs_bmap *bmap,
 				  struct buffer_head *bh)
 {
 	struct nilfs_direct *direct = (struct nilfs_direct *)bmap;