filesystems/mergerfs: fix build on 32 bit platforms

There was a type confusion in a compatibility stub. It accidentially
works on 64 bit platforms, but never on 32 bit platforms.

Approved by: portmgr (build fix blanket)
MFH: 2025Q3

(cherry picked from commit a4c0259a29c7cd77888805642fdc881e460a833d)