xfs: split out a btree type from the btree ops geometry flags

xfs: split out a btree type from the btree ops geometry flags

Source kernel commit: 4f0cd5a555072e21fb589975607b70798e073f8f

Two of the btree cursor flags are always used together and encode
the fundamental btree type.  There currently are two such types:

1) an on-disk AG-rooted btree with 32-bit pointers
2) an on-disk inode-rooted btree with 64-bit pointers

and we're about to add:

3) an in-memory btree with 64-bit pointers

Introduce a new enum and a new type field in struct xfs_btree_geom
to encode this type directly instead of using flags and change most
code to switch on this enum.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: make the pointer lengths explicit]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>