Backport !4019 “gunixmounts: Use fallback if libmount monitoring fails” to glib-2-80
The recently added libmount-based unix mount monitoring may fail when the
device exceeds inotify limits. Let's fallback to the older implementation
in case of the mnt_monitor_get_fd
function failure. This among others
fixes tracker-miners failures caused by seccomp rules.
Fixes: tracker-miners#315
Trivial backport of !4019 (merged) to glib-2-80
.