file backend: Explicitly cancel file monitor
If we don't cancel it before unreffing it, its last-unref might happen on the worker thread, which can lead to a deadlock.
Workaround for glib#1941 (closed).
Signed-off-by: Simon McVittie smcv@debian.org
Resolves: #226 (closed)
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=954197