btrfs: fix compiler warning on SPARC/PA-RISC handling fscrypt_setup_filename

Commit 1ec49744 ("btrfs: turn on -Wmaybe-uninitialized") exposed
that on SPARC and PA-RISC, gcc is unaware that fscrypt_setup_filename()
only returns negative error values or 0. This ultimately results in a
maybe-uninitialized warning in btrfs_lookup_dentry().

Change to only return negative error values or 0 from
fscrypt_setup_filename() at the relevant call site, and assert that no
positive error codes are returned (which would have wider implications
involving other users).
Reported-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/all/481b19b5-83a0-4793-b4fd-194ad7b978c3@roeck-us.net/Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -5421,8 +5421,13 @@ static int btrfs_inode_by_name(struct btrfs_inode *dir, struct dentry *dentry,
 		return -ENOMEM;
 
 	ret = fscrypt_setup_filename(&dir->vfs_inode, &dentry->d_name, 1, &fname);
-	if (ret)
+	if (ret < 0)
 		goto out;
+	/*
+	 * fscrypt_setup_filename() should never return a positive value, but
+	 * gcc on sparc/parisc thinks it can, so assert that doesn't happen.
+	 */
+	ASSERT(ret == 0);
 
 	/* This needs to handle no-key deletions later on */