ext4: kill ext4_mballoc_ready

This variable, introduced in commit 9c191f70

, is unnecessary: it is set
once the module has been initialized correctly, and ext4_fill_super
cannot run unless the module has been initialized correctly.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>