nfsd: fix dentry leak upon mkdir failure.

syzbot is reporting that nfsd_mkdir() forgot to remove dentry created by
d_alloc_name() when __nfsd_mkdir() failed (due to memory allocation fault
injection) [1].

[1] https://syzkaller.appspot.com/bug?id=ce41a1f769ea4637ebffedf004a803e8405b4674

Signed-off-by: Tetsuo Handa <[e-mail]>
Reported-by: syzbot <[e-mail]>
Fixes: e8a79fb14f6b76b5 ("nfsd: add nfsd/clients directory")
[bfields: clean up in nfsd_mkdir instead of __nfsd_mkdir]
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfsctl.c

···
 	inode_unlock(dir);
 	return dentry;
 out_err:
+	dput(dentry);
 	dentry = ERR_PTR(ret);
 	goto out;
 }