exfat: fix missing shutdown check

In the Linux kernel, the following vulnerability has been resolved:

exfat: fix missing shutdown check

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.