jbd2: don't unplug after writing revoke records

During commit process, keep the block device plugged after we are done
writing the revoke records, until we are finished writing the rest of
the commit records in the journal.  This will allow most of the
journal blocks to be written in a single I/O operation, instead of
separating the the revoke blocks from the rest of the journal blocks.

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