exfat: Month timestamp metadata accidentally incremented

The staging/exfat driver has departed, but a lot of distros are still tracking
5.4-stable, so we should fix this.

There was an 0/1 offset error in month handling for file metadata, causing
the month to get incremented on each reference to the file.

Thanks to Sebastian Gurtler for troubleshooting this, and Arpad Mueller
for bringing it to my attention.

Relevant discussions:
https://bugzilla.kernel.org/show_bug.cgi?id=210997
https://bugs.launchpad.net/ubuntu/+source/ubuntu-meta/+bug/1872504



Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>