uipc_shm.c: make large page allocation interruptible

in cases there is no page pressure or when the user lost patience
waiting for very large allocation. Other case is already handled by
vm_wait_intr().

Reported by: "Lizzie from Eden Emulator project"
Reviewed by: adrian, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D56725