If there are any directories with > 65000 subdirectories, enable the
DIR_NLINK feature in the superblock. If there are any directories
that formerly had > 65000 subdirs (i_links_count == 1) but no longer
do, don't consider this an error to alert the user about, but silently
fix the link count to the currently counted link count.
The DIR_NLINK feature is not disabled if set but no many-subdir directories
are found, so that the kernel is not required to enable it on-the-fly. The
admin should set it with tune2fs instead.