Invalid error code returned by MDS is causing a kernel client WARNING
Found by an occasionally failing xfstest generic/011.
After some investigation, it was found out that a positive error code EISDIR was being received by the kernel client in fs/namei.c atomic_open().
Issue seems to be fixed by https://github.com/ceph/ceph/pull/9693. After including this commit 100 runs of said xfstest were successful