exfat: add remount_fs operation for 5.4 kernel

The remount_fs operations should be defined for
kernel 5.4, because fs_context is not provided
for the 5.4 kernel and the operation will be
called when exfat is remounted.

This fixes xfstests generic/452 failure.

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>