ext4: fix hot spins in mballoc after err_freebuddy and err_freemeta

In ext4_mb_init_backend() 'i' is of type ext4_group_t. Since unsigned, i
>= 0 is always true, so fix hot spins after err_freebuddy: and -meta:
and prevent decrements when zero.

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>