ext4: Fix ext4_mb_initialize_context() to initialize all fields

Pavel Roskin pointed out that kmemcheck indicated that
ext4_mb_store_history() was accessing uninitialized values of
ac->ac_tail and ac->ac_buddy leading to garbage in the mballoc
history.  Fix this by initializing the entire structure to all zeros
first.

Also, two fields were getting doubly initialized by the caller of
ext4_mb_initialize_context, so remove them for efficiency's sake.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>