usb: gadget: f_mass_storage: call try_to_freeze only when its safe

Call try_to_freeze() in sleep_thread() only when it's safe to sleep.
do_read() and do_write() calls sleep_thread with lock held.
Make sure these won't call try_to_freeze() by passing can_freeze flag
to sleep_thread.

Calling try_to_freeze() with a lock hold was done since day one in
f_mass_storage but since commit 0f9548ca ("lockdep: check that no
locks held at freeze time") lockdep complains about it.

Signed-off-by: George Cherian <george.cherian@ti.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>