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.