nilfs2: fix 64-bit division operations in nilfs_bmap_find_target_in_group()

With the change to make inode->i_ino a u64, the build started failing on
32-bit ARM with:

    ERROR: modpost: "__aeabi_uldivmod" [fs/nilfs2/nilfs2.ko] undefined!

Fix this by using udiv_u64() for the division.

For finding the index into the group, switch to using a bitwise &
operation since that's more efficient. With this change however,
NILFS_BMAP_GROUP_DIV must be a power of two, so add a compile-time
assertion for that.

Fixes: 998a59d ("treewide: fix missed i_ino format specifier conversions")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603100602.KPxiClIO-lkp@intel.com/
Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260316-iino-u64-v3-2-d1076b8f7a20@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: jtlayton

fs/nilfs2/bmap.c

@@ -450,18 +450,25 @@ __u64 nilfs_bmap_find_target_seq(const struct nilfs_bmap *bmap, __u64 key)
 		return NILFS_BMAP_INVALID_PTR;
 }
 
-#define NILFS_BMAP_GROUP_DIV	8
+#define NILFS_BMAP_GROUP_DIV	8	/* must be power of 2 */
+
 __u64 nilfs_bmap_find_target_in_group(const struct nilfs_bmap *bmap)
 {
 	struct inode *dat = nilfs_bmap_get_dat(bmap);
 	unsigned long entries_per_group = nilfs_palloc_entries_per_group(dat);
-	unsigned long group = bmap->b_inode->i_ino / entries_per_group;
+	unsigned long group;
+	u32 index;
+
+	BUILD_BUG_ON_NOT_POWER_OF_2(NILFS_BMAP_GROUP_DIV);
+
+	group = div_u64(bmap->b_inode->i_ino, entries_per_group);
+	index = bmap->b_inode->i_ino & (NILFS_BMAP_GROUP_DIV - 1);
 
 	return group * entries_per_group +
-		(bmap->b_inode->i_ino % NILFS_BMAP_GROUP_DIV) *
-		(entries_per_group / NILFS_BMAP_GROUP_DIV);
+	       index * (entries_per_group / NILFS_BMAP_GROUP_DIV);
 }
 
+
 static struct lock_class_key nilfs_bmap_dat_lock_key;
 static struct lock_class_key nilfs_bmap_mdt_lock_key;