file: flush delayed work in delayed fput()

The fput() of file rcS might not have completed causing issues when
executing the file.

rcS is opened in do_populate_rootfs before executed. At the end of
do_populate_rootfs() flush_delayed_fput() is called. Now
do_populate_rootfs() assumes that all fput()s caused by
do_populate_rootfs() have completed.

But flush_delayed_fput() can only ensure that fput() on the current
delayed_fput_list has finished. Any file that has been removed from
delayed_fput_list asynchronously in the meantime might not have
completed causing the exec to fail.

do_populate_rootfs	delayed_fput_list	delayed_fput	execve
fput()			a
fput()			a->b
fput()			a->b->rcS
						__fput(a)
fput()			c
fput()			c->d
						__fput(b)
flush_delayed_fput
__fput(c)
__fput(d)
						__fput(b)
						__fput(b)	execve(rcS)

Ensure that all delayed work is done by calling flush_delayed_work() in
flush_delayed_fput() explicitly.

Signed-off-by: Chen Lin <chen.lin5@zte.com.cn>
Signed-off-by: Shao Mingyin <shao.mingyin@zte.com.cn>
Link: https://lore.kernel.org/r/20241023135850067m3w2R0UXESiVCYz_wdAoT@zte.com.cn


Cc: Yang Yang <yang.yang29@zte.com.cn>
Cc: Yang Tao <yang.tao172@zte.com.cn>
Cc: Xu Xin <xu.xin16@zte.com.cn>
[brauner: rewrite commit message]
Signed-off-by: Christian Brauner <brauner@kernel.org>