btrfs-progs: corrupt-block: fix a delete and use bug corrupting extent tree

btrfs-progs: corrupt-block: fix a delete and use bug corrupting extent tree

When corrupting extent tree, corrupt-block will iterate each child
node/leaf of a node.
However, when a node's child is leaf, btrfs_corrupt_extent_leaf() may
delete some item in the leaf, which may cause the children number of the
parent node decrease.

Before this patch, corrupt-block will read out the nritems only *ONCE*
and iterate the 'nritems' times.
When btrfs_corrupt_extent_leaf() deletes enough item, causing the
nritems of btrfs_header decreased, the last few iteration will access
non-existed node, which will cause the delete and use bug like
the following:

deleting extent record: key 40714240 168 16384
Couldn't map the block 3459802452797161472
btrfs-corrupt-block: volumes.c:1137: btrfs_num_copies: Assertion
`!(!ce)' failed.
Aborted

This patch will update the nritmes in each iteration to avoid the bug.

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