bcachefs: Write buffer flush needs BTREE_INSERT_NOCHECK_RW

btree write buffer flush is only invoked from contexts that already hold
a write ref, and checking if we're still RW could cause us to fail to
completely flush the write buffer when shutting down.

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

fs/bcachefs/btree_write_buffer.c

@@ -78,6 +78,7 @@ trans_commit:
 	return  bch2_trans_update(trans, iter, &wb->k, 0) ?:
 		bch2_trans_commit(trans, NULL, NULL,
 				  commit_flags|
+				  BTREE_INSERT_NOCHECK_RW|
 				  BTREE_INSERT_NOFAIL|
 				  BTREE_INSERT_JOURNAL_RECLAIM);
 }