ext4: remove redundant code in ext4_alloc_inode()

inode_init_always() will initialize inode->i_data.writeback_index
anyway, no need to do this in ext4_alloc_inode().

Signed-off-by: Guo Chao <yan@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>