e2fsck: speed up runs when using tdb for large atypical filesystems

e2fsck: speed up runs when using tdb for large atypical filesystems

Optimize how the tdb library so that running with [scratch_files] in
/etc/e2fsck.conf is more efficient.  Use a better hash function,
supplied by Rogier Wolff, and supply an estimate of the size of the
hash table to tdb_open instead of using the default (which is way too
small in most cases).  Also, disable the tdb locking and fsync calls,
since it's not necessary for our use in this case (which is
essentially as cheap swap space; the tdb files do not contain
persistent data.)

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>