Staging: android: task_get_unused_fd_flags: fix the wrong usage of tsk->signal

Compile tested.

task_struct->signal is not protected by RCU, the code is bogus.
Change the code to take ->siglock to pin ->signal.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Brian Swetland <swetland@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>