usb: gadget: f_fs: Fix use-after-free for unbind with remaining io
If usb has stall, then there can be remaining submitted io and
unbinding f_fs with the remaining io, there is use-after-free.
Fix the use-after-free by checking endpoint after wait.
This fixes following kasan warning:
BUG: KASAN: use-after-free in ffs_epfile_io+0x654/0xb58
Read of size 4 at addr
ffffffc0a44e65dc by task mtp-responder/5117
...
[<
ffffff900a037794>] ffs_epfile_io+0x654/0xb58
[<
ffffff900a03818c>] ffs_epfile_read_iter+0x1ac/0x3e0
...
Allocated by task 3869:
...
__kmalloc+0x234/0x760
_ffs_func_bind+0x264/0x7c8
ffs_func_bind+0xe8/0x650
usb_add_function+0x13c/0x378
...
Freed by task 3869:
...
kfree+0xa4/0x750
ffs_func_unbind+0x150/0x248
purge_configs_funcs+0x1a0/0x310
...
Change-Id: I2bb9b07d93b1ac42432caaa2c2176d987b36b140
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>