btrfs: move leak debug code to functions

btrfs: move leak debug code to functions

Clean up the leak debugging in extent_io.c by moving
the debug code into functions.  This also removes the
list_heads used for debugging from the extent_buffer
and extent_state structures when debug is not enabled.

Since we need a global debug config to do that last
part, implement CONFIG_BTRFS_DEBUG to accommodate.

Thanks to Dave Sterba for the Kconfig bit.

Signed-off-by: Eric Sandeen <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: Josef Bacik <[email protected]>