xfs: switch (back) to a per-buftarg buffer hash

xfs: switch (back) to a per-buftarg buffer hash

The per-AG buffer hashes were added when all buffer lookups took a
per-hash look.  Since then we've made lookups entirely lockless and
removed the need for a hash-wide lock for inserts and removals as
well.  With this there is no need to sharding the hash, so reduce the
used resources by using a per-buftarg hash for all buftargs.

Signed-off-by: Christoph Hellwig <hch@lst.de>