ext2: set proper errno in error case of ext2_fill_super()

Set proper errno in the case of failure of
initializing percpu variables.

Link: https://lore.kernel.org/r/20191129013636.7624-1-cgxu519@mykernel.net
Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Jan Kara <jack@suse.cz>