ext4: move 'ix' sanity check to corrent position

[ Upstream commit af90a8f4a09ec4a3de20142e37f37205d4687f28 ]

Check 'ix' before it is used.

Fixes: 80e675f9 ("ext4: optimize memmmove lengths in extent/index insertions")
Signed-off-by: Gou Hao <gouhao@uniontech.com>
Link: https://lore.kernel.org/r/20230906013341.7199-1-gouhao@uniontech.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>