xfs: remove a superflous hash lookup when inserting new buffers

Currently on the slow path insert we repeat the initial hash table
lookup before we attempt the insert, resulting in a two traversals
of the hash table to ensure the insert is valid. The rhashtable API
provides a method for an atomic lookup and insert operation, so we
can avoid one of the hash table traversals by using this method.

Adapted from a large patch containing this optimisation by Christoph
Hellwig.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>