Use SPP (Supervisor Previous Privilege) bit in the sstatus
register to determine if trap is from userspace.

Otherwise if we jump to kernel address from userspace, then
TRAPF_USERMODE failed to detect usermode and then do_ast
triggers a panic "ast in kernel mode".

Reviewed by: markj@
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16469