Btrfs: avoid risk of a deadlock in btrfs_handle_error

Remove the attempt to cancel a running scrub or device replace
operation in btrfs_handle_error() because it adds the risk of
a deadlock. The only penalty of not canceling the operation is
that some I/O remains active until the procedure completes.
This is basically the same thing that happens to other tasks
that are running in user mode context, they are not affected
or stopped in btrfs_handle_error(), these tasks just need to
handle write errors correctly.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>