btrfs: update stale comment at extent_io_tree_release()

btrfs: update stale comment at extent_io_tree_release()

There's this comment at extent_io_tree_release() that mentions io btrees,
but this function is no longer used only for io btrees. Originally it was
added as a static function named clear_btree_io_tree() at transaction.c,
in commit 663dfbb07774 ("Btrfs: deal with convert_extent_bit errors to
avoid fs corruption"), as it was used only for cleaning one of the io
trees that track dirty extent buffers, the dirty_log_pages io tree of a
a root and the dirty_pages io tree of a transaction. Later it was renamed
and exported and now it's used to cleanup other io trees such as the
allocation state io tree of a device or the csums range io tree of a log
root.

So remove that comment and replace it with one at the top of the function
that is more complete, mentioning what the function does and that it's
expected to be called only when a task is sure no one else will need to
use the tree anymore, as well as there should be no locked ranges in the
tree and therefore no waiters on its extent state records. Also add an
assertion to check that there are no locked extent state records in the
tree.

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