Change acl mount options handling to match the one of XFS and BTRFS and
hopefully it is also easier to use now. When admin does not specify any
acl mount option, acls are enabled if and only if the filesystem has
xattr feature enabled. If admin specifies 'acl' mount option, we fail
the mount if the filesystem does not have xattr feature and thus acls
cannot be enabled.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Joel Becker <joel.becker@oracle.com>