e2fsck: fix dangling pointer when dir_info array is resized

e2fsck: fix dangling pointer when dir_info array is resized

e2fsck uses an array to store directory usage information during pass
3; the usage context also contains a pointer to the last directory
looked up.  When expanding the dir_info array, this cache pointer
needs to be cleared if the array resize changed the pointer location,
or else we'll later walk off the end of this dead pointer.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reported-by: Sami Liedes <sami.liedes@iki.fi>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>