fs: use debug-only asserts around fd allocation and install

This also restores the check which got removed in 52732bb
("fs/file.c: remove sanity_check and add likely/unlikely in alloc_fd()")
for performance reasons -- they no longer apply with a debug-only
variant.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250312161941.1261615-1-mjguzik@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mjguzik

fs/file.c

@@ -577,6 +577,7 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
 
 	__set_open_fd(fd, fdt, flags & O_CLOEXEC);
 	error = fd;
+	VFS_BUG_ON(rcu_access_pointer(fdt->fd[fd]) != NULL);
 
 out:
 	spin_unlock(&files->file_lock);
@@ -642,15 +643,15 @@ void fd_install(unsigned int fd, struct file *file)
 		rcu_read_unlock_sched();
 		spin_lock(&files->file_lock);
 		fdt = files_fdtable(files);
-		WARN_ON(fdt->fd[fd] != NULL);
+		VFS_BUG_ON(rcu_access_pointer(fdt->fd[fd]) != NULL);
 		rcu_assign_pointer(fdt->fd[fd], file);
 		spin_unlock(&files->file_lock);
 		return;
 	}
 	/* coupled with smp_wmb() in expand_fdtable() */
 	smp_rmb();
 	fdt = rcu_dereference_sched(files->fdt);
-	BUG_ON(fdt->fd[fd] != NULL);
+	VFS_BUG_ON(rcu_access_pointer(fdt->fd[fd]) != NULL);
 	rcu_assign_pointer(fdt->fd[fd], file);
 	rcu_read_unlock_sched();
 }