libext2fs: fix tdb.c mmap leak

libext2fs: fix tdb.c mmap leak

When undoing an expansion of an mmap'd database while cancelling a
transaction, the tdb code prematurely decreases the variable that
tracks the file size, which leads to a region leak during the
subsequent unmap.  Fix this by maintaining a separate counter for the
region size.

(This is probably unnecessary since e2undo was the only user of tdb
transactions, but I suppose we could be proactive.)

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