ocfs2: Fix writeout in ocfs2_data_convert_worker()

Commit f1f540688eae66c274ff1c1133b5d9c687b28f58 "optimized"
ocfs2_data_convert_worker() to "only do work for regular files".
Unfortunately, I left out a '!', which casued it to *skip* regular files.
This was hidden from testing until recently because the default data
journaling mode (data=ordered) doesn't exercise this code.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>