xfs_repair: fix btree block magic number mapping

xfs_repair: fix btree block magic number mapping

The magic numbers for generic btree blocks were modified some time
ago (before the kernel code was committed) but the xfs_repair
mapping code was not updated to match. It's no longer a simple
mapping, so just make the code a dense array and use the magic
number as the search key.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>