usb: gadget: f_fs: Fix use-after-free for unbind with remaining io

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>