ext2: improve ext2_readdir() return value

Improve ext2_readdir() return value for ext2_get_page() failure by using the
actual result of ext2_get_page().

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>