fix: Disable inode cache if filesystem risks getting full soon

fix: Disable inode cache if filesystem risks getting full soon

Some filesystems, for instance btrfs with compression enabled,
apparently make a posix_fallocate call succeed without actually
allocating the requested space for the file. This means that if the file
is mapped into memory, like done by the inode cache, the process can
crash when accessing the memory if the filesystem is full.

This commit implements a workaround: the inode cache is disabled if the
filesystem reports that it has less than 100 MiB free space. The free
space check is valid for one second before it is done again. This should
hopefully make crashes very rare in practice.

Closes #1236.