Btrfs: Use map_private_extent_buffer during generic_bin_search

It is possible that generic_bin_search will be called on a tree block
that has not been locked.  This happens because cache_block_block skips
locking on the tree blocks.

Since the tree block isn't locked, we aren't allowed to change
the extent_buffer->map_token field.  Using map_private_extent_buffer
avoids any changes to the internal extent buffer fields.

Signed-off-by: Chris Mason <chris.mason@oracle.com>