ext4: reserve inodes and feature code for 'quota' feature

I am working on patch to add quota as a built-in feature for ext4
filesystem. The implementation is based on the design given at
https://ext4.wiki.kernel.org/index.php/Design_For_1st_Class_Quota_in_Ext4.
This patch reserves the inode numbers 3 and 4 for quota purposes and
also reserves EXT4_FEATURE_RO_COMPAT_QUOTA feature code.

Signed-off-by: Aditya Kali <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/ext4/ext4.h

···
  */
 #define	EXT4_BAD_INO		 1	/* Bad blocks inode */
 #define EXT4_ROOT_INO		 2	/* Root inode */
+#define EXT4_USR_QUOTA_INO	 3	/* User quota inode */
+#define EXT4_GRP_QUOTA_INO	 4	/* Group quota inode */
 #define EXT4_BOOT_LOADER_INO	 5	/* Boot loader inode */
 #define EXT4_UNDEL_DIR_INO	 6	/* Undelete directory inode */
 #define EXT4_RESIZE_INO		 7	/* Reserved group descriptors inode */
···
 #define EXT4_FEATURE_RO_COMPAT_GDT_CSUM		0x0010
 #define EXT4_FEATURE_RO_COMPAT_DIR_NLINK	0x0020
 #define EXT4_FEATURE_RO_COMPAT_EXTRA_ISIZE	0x0040
+#define EXT4_FEATURE_RO_COMPAT_QUOTA		0x0100
 
 #define EXT4_FEATURE_INCOMPAT_COMPRESSION	0x0001
 #define EXT4_FEATURE_INCOMPAT_FILETYPE		0x0002