btrfs: exit after state insertion failure at btrfs_convert_extent_bit()

In the Linux kernel, the following vulnerability has been resolved:

btrfs: exit after state insertion failure at btrfs_convert_extent_bit()

If insert_state() state failed it returns an error pointer and we call

extent_io_tree_panic() which will trigger a BUG() call. However if

CONFIG_BUG is disabled, which is an uncommon and exotic scenario, then

we fallthrough and call cache_state() which will dereference the error

pointer, resulting in an invalid memory access.

So jump to the 'out' label after calling extent_io_tree_panic(), it also

makes the code more clear besides dealing with the exotic scenario where

CONFIG_BUG is disabled.