jfs: Convert inc_io to take a folio

All their callers now have a folio, so pass it in.  Remove mp_anchor()
as inc_io() was the last user.  No savings here, just cleaning up some
remnants.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>