linux/futex: Don't load a timeout when try-locking a mutex

[markj]

Summary

linux_sys_futex() does not copyin a timespec for the timeout if the
operation is LINUX_FUTEX_TRYLOCK_PI, presumably because it doesn't make
sense to specify a timeout for a try-lock operation. However, this
means that we pass a userspace timespec pointer to
linux_umtx_abs_timeout_init().

Modify linux_futex_lock_pi() to not initialize the timeout if we're
try-locking.

[kib]

sys/compat/linux/linux_futex.c

372–373

the if (try) return EBUSY; below should avoid the use of uninitialized timo, but are all compilers smart enough to follow the control flow?
It took some time to understand that.

If we introduce a pointer to timo, and set it to NULL in the try || ts == NULL case, then we can remove the conditional from the line 549.

[markj]

Apply kib's suggestion