Btrfs: add datacow flag in inode flag

For datacow control, the corresponding inode flags are needed.
This is for btrfs use.

v1->v2:
Change FS_COW_FL to another bit due to conflict with the upstream e2fsprogs
Signed-off-by: NLiu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

include/linux/fs.h

@@ -364,6 +364,8 @@ struct inodes_stat_t {
 #define FS_TOPDIR_FL			0x00020000 /* Top of directory hierarchies*/
 #define FS_EXTENT_FL			0x00080000 /* Extents */
 #define FS_DIRECTIO_FL			0x00100000 /* Use direct i/o */
+#define FS_NOCOW_FL			0x00800000 /* Do not cow file */
+#define FS_COW_FL			0x02000000 /* Cow file */
 #define FS_RESERVED_FL			0x80000000 /* reserved for ext2 lib */
 
 #define FS_FL_USER_VISIBLE		0x0003DFFF /* User visible flags */