exfat: fix missing shutdown check

[ Upstream commit 47e35366 ]

xfstests generic/730 test failed because after deleting the device
that still had dirty data, the file could still be read without
returning an error. The reason is the missing shutdown check in
->read_iter.

I also noticed that shutdown checks were missing from ->write_iter,
->splice_read, and ->mmap. This commit adds shutdown checks to all
of them.

Fixes: f761fcdd ("exfat: Implement sops->shutdown and ioctl")
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>