kernel - Make adjustments to the per-lwp ucred
* Rename lwp_syscall_ucred to lwp_ucred and move ucred synchronization
from syscall2() to userenter(). It now runs on any user entry into
the kernel, not just for system calls.
NOTE: A VM fault from the kernel itself does not call userenter() and
does not trigger another synchronization. This is the behavior
we desire, to ensure that lwp_ucred is stable.
There are numerous non-syscall cases (e.g. signal, fault, etc) where
we may need to access a safe ucred.
* Remove the SMP conditional. lwp_ucred is now tracked for both UP and SMP.
There is no point optimizing this for UP since there is basically no cost
for tracking lwp_ucred.