nfs: return EISDIR on nfs3_proc_create if d_alias is a dir

In the Linux kernel, the following vulnerability has been resolved:

nfs: return EISDIR on nfs3_proc_create if d_alias is a dir

If we found an alias through nfs3_do_create/nfs_add_or_obtain

/d_splice_alias which happens to be a dir dentry, we don't return

any error, and simply forget about this alias, but the original

dentry we were adding and passed as parameter remains negative.

This later causes an oops on nfs_atomic_open_v23/finish_open since we

supply a negative dentry to do_dentry_open.

This has been observed running lustre-racer, where dirs and files are

created/removed concurrently with the same name and O_EXCL is not

used to open files (frequent file redirection).

While d_splice_alias typically returns a directory alias or NULL, we

explicitly check d_is_dir() to ensure that we don't attempt to perform

file operations (like finish_open) on a directory inode, which triggers

the observed oops.