bcachefs: Delete an incorrect bch2_trans_unlock()

bcachefs: Delete an incorrect bch2_trans_unlock()

These deletes a bch2_trans_unlock() call from __bch2_move_data(). It was
redundant; bch2_move_extent() has the correct unlock call, and it was
buggy because when move_extent calls bch2_extent_drop_ptrs() we don't
want the transaction to be unlocked yet - this fixes a btree_iter.c
assertion.

Fixes https://github.com/koverstreet/bcachefs/issues/511.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>