xfs: don't decrement the buffer LRU count for in-use buffers

xfs: don't decrement the buffer LRU count for in-use buffers

XFS buffers are added to the LRU when they are unused, but are only
removed from the LRU lazily when the LRU list scan finds a used buffer.
So far this only happen when the LRU counter hits 0, which is suboptimal
as buffers that were added to the LRU, but are in use again still consume
LRU scanning resources and are aged while actually in use.

Fix this by checking for in-use buffers and removing the from the LRU
before decrementing the LRU counter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>