[XFS] Convert l_flushsema to a sv_t

The l_flushsema doesn't exactly have completion semantics, nor mutex
semantics. It's used as a list of tasks which are waiting to be notified
that a flush has completed. It was also being used in a way that was
potentially racy, depending on the semaphore implementation.

By using a sv_t instead of a semaphore we avoid the need for a separate
counter, since we know we just need to wake everything on the queue.

Original waitqueue implementation from Matthew Wilcox. Cleanup and
conversion to sv_t by Christoph Hellwig.

SGI-PV: 981507
SGI-Modid: xfs-linux-melb:xfs-kern:31059a

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>