undo-io: use a bitmap to track what we've already written

undo-io: use a bitmap to track what we've already written

It's really inefficient to (ab)use the TDB key store as a bitmap to
find out if we've already written a block to the undo file, because
the tdb code is reads the database key btree disk blocks for *every*
query.  Changing that logic to a bitmap reduces overhead by a large
margin -- the overhead of using undo_io while converting a 2TB FS to
metadata_csum is reduced from 55 minutes to 45.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>