ext4: miscellaneous error checks and coding cleanups for mballoc

ext4_mb_seq_history_open(): check if sbi->s_mb_history is NULL

ext4_mb_history_init(): replace kmalloc and memset with kzalloc

ext4_mb_init_backend(): remove memset since kzalloc is used

ext4_mb_init(): the return value of ext4_mb_init_backend is int,
	but i is unsigned, replace it with a new int variable.

Signed-off-by: Shen Feng <shen@cn.fujitsu.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>