exfat: combine iocharset and utf8 option setup

Currently, exfat utf8 mount option depends on the iocharset option
value. After exfat remount, utf8 option may become inconsistent with
iocharset option.

If the options are inconsistent; (specifically, iocharset=utf8 but
utf8=0) readdir may reference uninitalized NLS, leading to a null
pointer dereference.

Extract and combine utf8/iocharset setup logic into exfat_set_iocharset().
Then Replace iocharset setup logic to exfat_set_iocharset to prevent
utf8/iocharset option inconsistentcy after remount.

Reported-by:  <syzbot+3e9cb93e3c5f90d28e19@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=3e9cb93e3c5f90d28e19


Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Fixes: acab02ffcd6b ("exfat: support modifying mount options via remount")
Tested-by:  <syzbot+3e9cb93e3c5f90d28e19@syzkaller.appspotmail.com>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>