signal: Hand SIGQUEUE_PREALLOC flag to __sigqueue_alloc()

There is no point in having the conditional at the callsite.

Just hand in the allocation mode flag to __sigqueue_alloc() and use it to
initialize sigqueue::flags.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210322092258.898677147@linutronix.de