Ext2: remove the overhead check about sb in the function ext2_new_blocks

It can be guranteed that inode->i_sb should not be null in vfs.
So here the check about it is overhead.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: Jan Kara <jack@suse.cz>