xfstests: add test to reproduce bmap btree corruption.

xfstests: add test to reproduce bmap btree corruption.

The problem was reprted here:

https://bugzilla.redhat.com/show_bug.cgi?id=626244

With the simple test case:

# mkfs.xfs -f -d agsize=16m,size=50g <dev>
# mount <dev> /mnt
# xfs_io -f -c 'resvsp 0 40G' /mnt/foo

Triggering the problem. Turn this into a new xfsqa test so that we
exercise the problem code and prevent future regressions.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>