Input: fix locking issue in /proc/bus/input/ handlers

input_devices_seq_start() uses mutex_lock_interruptible() to acquire
the input_mutex, but doesn't properly handle the situation when the
call fails (for example due to interrupt). Instead of returning NULL
(which indicates that there is no more data) we should return
ERR_PTR()-encoded error.

We also need explicit flag indicating whether input_mutex was acquired
since input_devices_seq_stop() is called whether input_devices_seq_start()
was successful or not.

The same applies to input_handlers_seq_start().

Reported-by: iceberg <strakh@ispras.ru>
Reviewed-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>