undo-io: write out index block after every write

undo-io: write out index block after every write

Write out the undo file's index block after writing a block to the
undo file.  This ensures that we always have a consistent undo file
in the page cache, even if the program crashes.  When we fill up a
key block in the undo file, we'll call fsync to force the whole
thing to storage; this should happen about every 256 blocks given
the usual 4K block size.

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