e2fsck: set dir_nlink feature if large dir exists

e2fsck: set dir_nlink feature if large dir exists

If there is a directory with more than EXT2_LINK_MAX (65000)
subdirectories, but the DIR_NLINK feature is not set in the
superblock, the feature should be set before continuing on
to change the on-disk directory link count to 1.

While most filesystems should have DIR_NLINK set (it was set
by default for all ext4 filesystems, and all kernels between
2.6.23 and 4.12 automatically set it if the directory link
count grew too large), it is possible that this flag is lost
due to disk corruption or for an upgraded filesystem.  We no
longer want kernels to automatically enable features.

Addresses: https://bugzilla.kernel.org/show_bug.cgi?id=196405
Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>