usb: fix adb reboot panic.

usb: fix adb reboot panic.

PD#146539: usb: fix adb reboot panic.

Avoid kernel panic caused by race condition. For example,
1. In the ffs_epfile_io function, data buffer is allocated
for non-halt requests and the address of this buffer is
writed to usb controller registers.
2. After adb process be killed, data buffer is freed and
this memory is allocated for the other. But the address
is hold by the controller.
3. Adbd in PC is running. So, the controller receive the
data and write to this memory.
4. The value of this memory is modified by the controller.
This could cause the kernel panic.

To avoid this, during FunctionFS mount, we allocated the
data buffer for requests. And the memory resources has
been released in kill_sb.

Change-Id: Ie06fae8ce18ea553d71f4841458c3c3af096ff4b
Signed-off-by: Yue Wang <yue.wang@amlogic.com>