xfs: scrub the shape of a metadata btree

xfs: scrub the shape of a metadata btree

Source kernel commit: cc3e0948d2686f30f49166660cf85b7e0194f365

Create a function that can check the shape of a btree -- each block
passes basic inspection and all the pointers look ok.  In the next patch
we'll add the ability to check the actual keys and records stored within
the btree.  Add some helper functions so that we report detailed scrub
errors in a uniform manner in dmesg.  These are helper functions for
subsequent patches.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>