ext4: code cleanup for get_next_id

Now the checks in ext4_get_next_id() and dquot_get_next_id()
are almost the same, so just call dquot_get_next_id() instead
of ext4_get_next_id().

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Link: https://lore.kernel.org/r/20191006103028.31299-1-cgxu519@mykernel.net


Signed-off-by: Theodore Ts'o <tytso@mit.edu>