sys/vfs/hammer: Don't fail to downgrade cursor

sys/vfs/hammer: Don't fail to downgrade cursor

7ef2d7b should have placed end: before
hammer_cursor_downgrade(cursor);.

If hammer volume-del starts to reblock because of non-empty
volume, and first reblocks data, but skips reblock for btree
node as the node was not in that volume (data and btree node
stored in different volume which is usually a rare case),
then volume-del returns from hammer_reblock_helper() without
donwgrading upgraded lock.

This could happen only on volume-del, but not hammer reblock
command since hammer reblock command never hits goto end;.