ext2: code cleanup for ext2_try_to_allocate()

Code cleanup by removing duplicated code.

Link: https://lore.kernel.org/r/20191104114036.9893-4-cgxu519@mykernel.net


Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Jan Kara <jack@suse.cz>