btrfs: Fix double-free in Inode::_Read().

extent_data is now held by a MemoryDeleter, so thus with this line
it would have been freed twice.

src/add-ons/kernel/file_systems/btrfs/Inode.cpp

@@ -347,7 +347,6 @@ Inode::ReadAt(off_t pos, uint8* buffer, size_t* _length)
 		panic("unknown extent compression; %d\n", compression);
 		return B_BAD_DATA;
 	}
-	free(extent_data);
 	return B_OK;
 
 }