xfs: support in-memory btrees

Adapt the generic btree cursor code to be able to create a btree whose
buffers come from a (presumably in-memory) buftarg with a header block
that's specific to in-memory btrees.  We'll connect this to other parts
of online scrub in the next patches.

Note that in-memory btrees always have a block size matching the system
memory page size for efficiency reasons.  There are also a few things we
need to do to finalize a btree update; that's covered in the next patch.

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

fs/xfs/xfs_trace.c

@@ -37,6 +37,7 @@
 #include <linux/iomap.h>
 #include "xfs_iomap.h"
 #include "xfs_buf_mem.h"
+#include "xfs_btree_mem.h"
 
 /*
  * We include this last to have the helpers above available for the trace