jbd: clear b_modified before moving the jh to a different transaction

jbd: clear b_modified before moving the jh to a different transaction

journal_forget() and journal_invalidatepage() functions move buffer to
BJ_Forget list of a running transaction so that the buffer gets cleaned up when
the transaction is committed. This usually happens when underlying block is
freed but journal_invalidatepage() can also move the buffer when page cache of
the corresponding inode (may be a block device) gets flushed.  When the buffer
had b_modfied set from the previous transaction and we happen to modify it
again in the current transaction, we won't properly account for the modified
buffer by subtracting the number of reserved credits of the running transaction
because do_get_write_access() won't clear b_modified (buffer already is on
running transaction so do_get_write_access() things it has nothing to do).
This then results in assertion failure in commit code because the transaction
has more buffers than reserved credits (t_nr_buffers > t_outstanding_credits).

We fix the issue by clearing b_modified before moving buffer to a BJ_Forget list
of another transaction because logically, it's not changed for that transaction
anymore.

CC: stable@kernel.org
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>