[XFS] use scalable vmap API

Implement XFS's large buffer support with the new vmap APIs. See the vmap
rewrite (db64fe02

) for some numbers. The biggest improvement that comes from
using the new APIs is avoiding the global KVA allocation lock on every call.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>