ext4: rearrange initialization to fix EXT4FS_DEBUG

The EXT4FS_DEBUG is a *very* developer specific #ifdef designed for
ext4 developers only.  (You have to modify fs/ext4/ext4.h to enable
it.)

Rearrange how we initialize data structures to avoid calling
ext4_count_free_clusters() until the multiblock allocator has been
initialized.

This also allows us to only call ext4_count_free_clusters() once, and
simplifies the code somewhat.

(Thanks to Chen Gang <gang.chen.5i5j@gmail.com> for pointing out a
!CONFIG_SMP compile breakage in the original patch.)

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>