btrfs-progs: Recover btree by dropping corrupted leaf/node.

btrfs-progs: Recover btree by dropping corrupted leaf/node.

Current btrfsck can skip corrupted leaf and even repair some corrupted
one if their bytenr or key order is wrong.

However when it comes to csum error leaf, btrfsck can only skip them,
which is OK for read-only iteration, but is bad for write.

This patch introduce the new repair_btree() function to recover the
btree, deleting all the corrupted leaf/node including corresponding
extent, allowing later write into the btree.

This patch provides the basis for later recovery with corrupted leaves.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>