Btrfs: fix tree mod log rewinded level and rewinding of moved keys

When we rewind REMOVE_WHILE_FREEING operations, there's code that allocates
a fresh buffer instead of cloning the old one. Setting that buffer's level
correctly was missing in this case.

When rewinding a MOVE_KEYS operation, btrfs_node_key_ptr_offset(slot) was
missing for memmove_extent_buffer()'s arguments.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>