writeback: disassociate inodes from dying bdi_writebacks

For the purpose of foreign inode detection, wb's (bdi_writeback's) are
identified by the associated memcg ID.  As we create a separate wb for
each memcg, this is enough to identify the active wb's; however, when
blkcg is enabled or disabled higher up in the hierarchy, the mapping
between memcg and blkcg changes which in turn creates a new wb to
service the new mapping.  The old wb is unlinked from index and
released after all references are drained.  The foreign inode
detection logic can't detect this condition because both the old and
new wb's point to the same memcg and thus never decides to move inodes
attached to the old wb to the new one.

This patch adds logic to initiate switching immediately in
wbc_attach_and_unlock_inode() if the associated wb is dying.  We can
make the usual foreign detection logic to distinguish the different
wb's mapped to the memcg but the dying wb is never gonna be in active
service again and there's no point in tracking the usage history and
reaching the switch verdict after enough data points are collected.
It's already known that the wb has to be switched.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>