FFS: remove ffs_fsfail_task

Now that dounmount() supports a dedicated taskqueue, we can simply call
it with MNT_DEFERRED directly from the failing context. This also
avoids blocking taskqueue_thread with a potentially-expensive unmount
operation.

Reviewed by: kib, mckusick
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D31016