Mitigate several problems with the softdep_request_cleanup() on busy
host.

Problems start appearing when there are several threads all doing
operations on a UFS volume and the SU workqueue needs a cleanup. It is
possible that each thread calling softdep_request_cleanup() owns the
lock for some dirty vnode (e.g. all of them are executing mkdir(2),
mknod(2), creat(2) etc) and all vnodes which must be flushed are locked
by corresponding thread. Then, we get all the threads simultaneously
entering softdep_request_cleanup().

There are two problems:

• Several threads execute MNT_VNODE_FOREACH_ALL() loops in parallel. Due to the locking, they quickly start executing 'in phase' with the speed of the slowest thread.
• Since each thread already owns the lock for a dirty vnode, other threads non-blocking attempt to lock the vnode owned by other thread fail, and loops executing without making the progress.

Retry logic does not allow the situation to recover. The result is
a livelock.

Fix these problems by making the following changes:

• Allow only one thread to enter MNT_VNODE_FOREACH_ALL() loop per mp. A new flag FLUSH_RC_ACTIVE guards the loop.
• If there were failed locking attempts during the loop, abort retry even if there are still work items on the mp work list. An assumption is that the items will be cleaned when other thread either fsyncs its vnode, or unlock and allow yet another thread to make the progress.

It is possible now that some calls would get undeserved ENOSPC from
ffs_alloc(), because the cleanup is not aggressive enough. But I do
not see how can we reliably clean up workitems if calling
softdep_request_cleanup() while still owning the vnode lock. I thought
about scheme where ffs_alloc() returns ERESTART and saves the retry
counter somewhere in struct thread, to return to the top level, unlock
the vnode and retry. But IMO the very rare (and unproven) spurious
ENOSPC is not worth the complications.

Reported and tested by: pho
Style and comments by: mckusick
Reviewed by: mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks