btrfs: fix uninitialized variable warnings in __set_extent_bit and convert_extent_bit

We will pass in the parent and p pointer into our tree_search function
to avoid doing a second search when inserting a new extent state into
the tree.  However because this is conditional upon passing in these
pointers the compiler seems to think these values can be uninitialized
if we're using -Wmaybe-uninitialized.  Fix this by initializing these
values.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>