tune2fs: Add support for turning on quota feature

tune2fs: Add support for turning on quota feature

This patch adds support for setting the quota feature in superblock
and allows selectively creating quota inodes (user or group or both)
in the superblock. Currently, modifying the quota feature is only
supported when the filesystem is unmounted.
Also, when setting the quota feature, tune2fs will use aquota.user or
aquota.group file inode number in superblock if these files exist.
Otherwise it will initialize empty quota inodes #3 and #4 and use them.

Here is how it works:
 # Set quota feature and initialize both (user and group) quota inodes
 $ tune2fs -O quota /dev/ram1

 # Enable only one type of quota
 $ tune2fs -Q usrquota /dev/ram1

 # Enable grpquota, disable usrquota
 $ tune2fs -Q ^usrquota,grpquota /dev/ram1

 # Clear quota feature and remove quota inodes
 $ tune2fs -O ^quota /dev/ram1

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