ext4: fix to check return value of freeze_bdev() in ext4_shutdown()

freeze_bdev() can fail due to a lot of reasons, it needs to check its
reason before later process.

Fixes: 783d9485 ("ext4: add EXT4_IOC_GOINGDOWN ioctl")
Cc: stable@kernel.org
Signed-off-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230606073203.1310389-1-chao@kernel.org


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