Add a .indx. xattr tag which adds the inode to an index of inodes keyed
by the hash of xattr names. An ioctl is added which then returns all
the inodes which may contain an xattr of the given name. Dropping all
xattrs now has to parse the name to find out if it also has to delete an
index item.
Signed-off-by: Zach Brown <zab@versity.com>