ext4: use kmem_cache_zalloc() in ext4_init_io_end()

Use advantage of kmem_cache_zalloc() to remove a memset() call in
ext4_init_io_end() and save a few bytes.

Before:
 [jj@dragon linux-2.6]$ size fs/ext4/page-io.o
    text    data     bss     dec     hex filename
    3016       0     624    3640     e38 fs/ext4/page-io.o
After:
 [jj@dragon linux-2.6]$ size fs/ext4/page-io.o
    text    data     bss     dec     hex filename
    3000       0     624    3624     e28 fs/ext4/page-io.o

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>