bcachefs: Kill read lock dropping in bch2_btree_node_lock_write_nofail()

dropping read locks in bch2_btree_node_lock_write_nofail() dates from
before we had the cycle detector; we can now tell the cycle detector
directly when taking a lock may not fail because we can't handle
transaction restarts.

This is needed for adding should_be_locked asserts.

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