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 <adityakali@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>