NFS: Fix leak of ctx->nfs_server.hostname

If userspace passes an nfs_mount_data struct in the data argument of
mount(2), then nfs23_parse_monolithic() or nfs4_parse_monolithic()
will allocate memory for ctx->nfs_server.hostname.  This needs to be
freed in nfs_parse_source(), which also allocates memory for
ctx->nfs_server.hostname, otherwise a leak will occur.

Reported-by: syzbot+193c375dcddb4f345091@syzkaller.appspotmail.com
Fixes: f2aedb71 ("NFS: Add fs_context support.")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/fs_context.c

@@ -832,6 +832,8 @@ static int nfs_parse_source(struct fs_context *fc,
 	if (len > maxnamlen)
 		goto out_hostname;
 
+	kfree(ctx->nfs_server.hostname);
+
 	/* N.B. caller will free nfs_server.hostname in all cases */
 	ctx->nfs_server.hostname = kmemdup_nul(dev_name, len, GFP_KERNEL);
 	if (!ctx->nfs_server.hostname)