nfsd: check for EBUSY from vfs_rmdir/vfs_unink.

vfs_rmdir and vfs_unlink can return -EBUSY if the
target is a mountpoint.  This currently gets passed to
nfserrno() by nfsd_unlink(), and that results in a WARNing,
which is not user-friendly.

Possibly the best NFSv4 error is NFS4ERR_FILE_OPEN, because
there is a sense in which the object is currently in use
by some other task.  The Linux NFSv4 client will map this
back to EBUSY, which is an added benefit.

For NFSv3, the best we can do is probably NFS3ERR_ACCES, which isn't
true, but is not less true than the other options.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfsd.h

@@ -281,6 +281,7 @@ void		nfsd_lockd_shutdown(void);
 #define nfserr_offload_denied		cpu_to_be32(NFS4ERR_OFFLOAD_DENIED)
 #define nfserr_wrong_lfs		cpu_to_be32(NFS4ERR_WRONG_LFS)
 #define nfserr_badlabel			cpu_to_be32(NFS4ERR_BADLABEL)
+#define nfserr_file_open		cpu_to_be32(NFS4ERR_FILE_OPEN)
 
 /* error codes for internal use */
 /* if a request fails due to kmalloc failure, it gets dropped.

fs/nfsd/vfs.c

@@ -1815,7 +1815,17 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
 out_drop_write:
 	fh_drop_write(fhp);
 out_nfserr:
+	if (host_err == -EBUSY) {
+		/* name is mounted-on. There is no perfect
+		 * error status.
+		 */
+		if (nfsd_v4client(rqstp))
+			err = nfserr_file_open;
+		else
+			err = nfserr_acces;
+	} else {
 		err = nfserrno(host_err);
+	}
 out:
 	return err;
 }