btrfs-progs: extent_io: Fix NULL pointer dereference in free_extent_buffer_final()

btrfs-progs: extent_io: Fix NULL pointer dereference in free_extent_buffer_final()

In free_extent_buffer_final() we access eb->tree->cache_size in
BUG_ON().  However eb->tree can be NULL if it's a cloned extent buffer.

Currently the cloned extent buffer is only used in backref.c,
paths_from_inode() function.  Thankfully that function is not used yet
(but could be pretty useful to convert inode number to path, so I'd like
to keep such function).

Anyway, check eb->tree before accessing its member.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>