ext2: return error when fail to allocating memory in ioctl

Currently, we do not check memory allocation
result for ei->i_block_alloc_info in ioctl,
this patch checks it and returns error in
failure case.

Link: https://lore.kernel.org/r/20191023135643.28837-1-cgxu519@mykernel.net
Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Jan Kara <jack@suse.cz>