ext4: use kzalloc in ext4_kzalloc()

Commit 9933fc0i (ext4: introduce ext4_kvmalloc(), ext4_kzalloc(), and
ext4_kvfree()) intruduced wrappers around k*alloc/vmalloc but introduced
a typo for ext4_kzalloc() by not using kzalloc() but kmalloc().

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>