Btrfs: fix regression in scrub path resolving

Btrfs: fix regression in scrub path resolving

commit 7a3ae2f8c8c8432e65467b7fc84d5deab04061a0 upstream.

In commit 4692cf58 we introduced new backref walking code for btrfs. This
assumes we're searching live roots, which requires a transaction context.
While scrubbing, however, we must not join a transaction because this could
deadlock with the commit path. Additionally, what scrub really wants to do
is resolving a logical address in the commit root it's currently checking.

This patch adds support for logical to path resolving on commit roots and
makes scrub use that.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>