f2fs: add a sysfs entry to show max open zones

This patch adds a sysfs entry showing the max zones that F2FS can write
concurrently.

Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: Yongpeng Yang

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -643,6 +643,12 @@ Contact:	"Jaegeuk Kim" <jaegeuk@kernel.org>
 Description:	Shows the number of unusable blocks in a section which was defined by
 		the zone capacity reported by underlying zoned device.
 
+What:		/sys/fs/f2fs/<disk>/max_open_zones
+Date:		November 2025
+Contact:	"Yongpeng Yang" <yangyongpeng@xiaomi.com>
+Description:	Shows the max number of zones that F2FS can write concurrently when a zoned
+		device is mounted.
+
 What:		/sys/fs/f2fs/<disk>/current_atomic_write
 Date:		July 2022
 Contact:	"Daeho Jeong" <daehojeong@google.com>

fs/f2fs/sysfs.c

@@ -1213,6 +1213,7 @@ F2FS_SBI_GENERAL_RW_ATTR(last_age_weight);
 F2FS_SBI_GENERAL_RW_ATTR(max_read_extent_count);
 #ifdef CONFIG_BLK_DEV_ZONED
 F2FS_SBI_GENERAL_RO_ATTR(unusable_blocks_per_sec);
+F2FS_SBI_GENERAL_RO_ATTR(max_open_zones);
 F2FS_SBI_GENERAL_RW_ATTR(blkzone_alloc_policy);
 #endif
 F2FS_SBI_GENERAL_RW_ATTR(carve_out);
@@ -1388,6 +1389,7 @@ static struct attribute *f2fs_attrs[] = {
 #endif
 #ifdef CONFIG_BLK_DEV_ZONED
 	ATTR_LIST(unusable_blocks_per_sec),
+	ATTR_LIST(max_open_zones),
 	ATTR_LIST(blkzone_alloc_policy),
 #endif
 #ifdef CONFIG_F2FS_FS_COMPRESSION