btrfs-progs: check: supplement extent backref list with rbtree

btrfs-progs: check: supplement extent backref list with rbtree

For the pathlogical case, like xfstests generic/297 that creates a
large file consisting of one, repeating reflinked extent, fsck can
take hours.  The root cause is that calling find_data_backref while
iterating the extent records is an O(n^2) algorithm.  For my
example test run, n was 2*2^20 and fsck was at 8 hours and counting.

This patch supplements the list with an rbtree and drops the runtime
of that testcase to about 20 seconds.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>