btrfs: remove unnecessary NULL checks before freeing extent state

btrfs: remove unnecessary NULL checks before freeing extent state

When manipulating extent bits for an io tree range, we have this pattern:

   if (prealloc)
          btrfs_free_extent_state(prealloc);

but this is not needed nowadays since btrfs_free_extent_state() ignores
a NULL pointer argument, following the common pattern of kernel and btrfs
freeing functions, as well as libc and other user space libraries.
So remove the NULL checks, reducing source code and object size.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>